Introduction

Evaporative cooling is a natural cooling process that utilizes the principle of evaporation to lower the temperature of the surrounding air or liquid. When water or another suitable liquid evaporates, it absorbs heat from the surrounding air or environment, resulting in a cooling effect. This process occurs as the more energetic molecules near the liquid's surface escape into the air, leaving behind the slower-moving molecules, which results in a decrease in temperature of air and increasing its humidity [1]. Various parameters influence the performance of the evaporative cooling process such as pad material and configuration, flow rate of water and air, outside atmospheric conditions such as relative humidity and temperature.[2, 3]. Out of the above parameters, pad material and configuration are the most vital parameter that has a significant influence on the evaporative cooler’s performance.

Basic function of the pads is to hold water and air and allow the energy exchange between them. It increases the exposed surface area for the energy exchange between the two fluids. Pad material, wettability, surface area and pore size are the various factors to be considered during the selection of a pad material. [4, 5]. Several researchers have conducted experiments using direct evaporative coolers to reduce the air temperature. Many researchers have used commercially available celdek packing as the filling material and have conducted experimental analysis in a single stage cooler[6, 7].They observed a good humidification efficiency with reasonable drop in the DBT at the exit. Several modifications have been proposed by various researchers that include changing the packing dimensions, configurations, materials and other operating conditions related to water and air[8]. Even the analysis using different organic biomass-based alternative materials have been performed[9,10,11]. But it was considered that the performance of the system was less than the commercially available celdek material. In addition, the air quality was also poor. But the system was economical with lower initial and operational costs. However, maintenance cost with such materials was higher.

In all the proposed models, for a given area of the packing, evaporation rate and drop in temperature were proportional which means that for a huge drop in the temperature, inevitably the rise in humidity is also huge, which is not desired. For thermal comfort, the relative humidity of air should be between 60–80%. Higher humidity will lead to the growth of microbes which may cause various health issues in people inhaling the humid air. On the other hand, it is always desired to have maximum reduction in the air temperature without appreciable increase in the exit humidity. To address this issue, different researchers have modified the basic design of direct evaporative coolers [1, 12, 13].

Heidarinjad et al. [14] experimentally investigated cooling efficiency of two-stage indirect–direct evaporative cooler for different climatic conditions. The working of two-stage evaporative cooler is such that at first, air is passed over IEC (Indirect evaporative cooler) unit then it is passed over DEC (Direct evaporative cooler). It was concluded that two-stage evaporative cooler gave better comfort than DEC alone. However, it was noted that the two-stage system consumes more water than DEC by around 55%. Zhou et al. [15] developed an experimental setup of two-stage indirect-thermoelectrically assisted direct evaporative cooler. Setup consisted of a cross flow regenerative IEC and thermoelectrically assisted DEC which served as the second stage heat exchanger. Air was cooled in IEC then passed over thermoelectrically assisted direct evaporative cooler for further cooling. Result showed that outlet air temperature and relative humidity could be improved by using an indirect evaporative cooler (IEC) A numerical model was generated and also validated by the experimental results. Imthiyaz et al.[16] constructed a numerical model with Gaussian process coupling algorithm for predicting the influence of meteorological parameters on the performance of an evaporative cooling system. They reported a good accuracy with R2 = 0.999 for the test data.

Farahani et al. [17] constructed a two-stage system consisting of an indirect evaporative cooling and nocturnal radiative cooling system. They concluded that hybrid systems have much more effectiveness than IEC alone. Heidarinejad et al. [18] found out the effect of spray water temperature variation along the surface of the heat exchanger while considering wall longitudinal heat conduction in a two-stage evaporative cooler. The finite difference method was used to discretize the governing equations and the discrete equation system was solved using an iterative method. Result showed that two-stage direct/indirect had 50% higher wet bulb effectiveness than the single stage with the same inlet parameters. Numerical results and experimental results agreed with each other with around 3% error. Dessouky et al. [19] analyzed the performance of a two-stage direct indirect evaporative cooler. High-density polythene was used as the packing material for direct evaporative cooler. Parameters like air flow rate outside heat exchanger, number of heat exchanger, water flow direction, water flow rate, thickness of packing were considered for experimentation. Results showed that the efficiency of direct–indirect cooler varied over a range of 90–120%, indicating that the dry bulb temperature of outlet air was lower than wet bulb temperature of the intake air. Alklaibi et al. [20] conducted an experimental investigation on an internal two-stage evaporative cooler. They used cellulose pad as cooling pad material for direct evaporative cooler with Maisotsenko cycle (M-cycle)-based evaporative cooler. The internal two-stage evaporative cooler had a greater system efficiency than the direct evaporative cooler unit, according to the results. When compared to a two-stage evaporative cooler, the performance of the DEC unit varied considerably more than when the air speed was varied. Since high relative humidity was observed with internal two-stage evaporative cooler, they were not recommended for very dry climate. But these were preferred in places where high humidity is looked for.

Dehumidification-evaporative cooling performance of an alumino-phosphate zeolite coated cross flow heat exchanger was studied by Sunhor et al. [21] wherein they observed that evaporative cooler’s performance was drastically deteriorated with the increase in entry water temperature above 55 °C. Performance of a hybrid evaporative cooler with latent heat thermal energy storage containing spherical balls with PCM was investigated by Wan Y. et al. [22]. They concluded that hybrid cooling offers better cooling due to the precooling effect on the primary air. Lai et al.[23] proposed a solar assisted solid desiccant unit integrated with conventional evaporative cooling. From the experiments, they claimed that by selecting a proper air recirculation rate, units can be used even in hot and humid climates, when only direct evaporative coolers will fail. Blanco-Marigorta et al. [24] conducted an experimental analysis on a multistage evaporative cooler using a heat pipe. Setup consisted of a direct and an indirect evaporative cooler unit. There were 2 types of modules for the arrangement of heat pipes. In the first type, air would get cooled before it enters the pad material. The second module was constructed in such a way that it cooled down the air produced using the output air coming out of the evaporative cooler. Result highlighted that the evaporative cooler and the multistage direct evaporative cooler with the heat pipe as a pre-cooler had a higher product air relative humidity than the evaporative cooler with a heat pipe heat exchanger. Even though the system performed better with first type module, there was a significant rise in its relative humidity. It was also observed that the second heat pipe module did not perform well compared to other arrangements.

It is always required to enhance the thermal performance of a heat exchanger by increasing the heat transfer rate. Various techniques have been used by the researchers over the years to augment the heat transfer rate [25]. Different power-induced active techniques and passive techniques which does not require the external energy have been suggested by the researchers[26, 27].

Al obaidi et al.[28] used interrupted groove patterns to modify the flow structure in a circular pipe. They found a higher performance in terms on Nu for the grooved tube as compared to the plain normal tube. Enhancement in the heat transfer rate has been numerically studied by Al obaidi et al.[29] using a dimpled twisted tape insert in a tubular heat exchanger. It performed well in comparison with a plain tube. The combination of twisted tape insert with a dimple shaped insert on the pipe surface led to an increased thermal performance as compared with a plain tube was observed by Alhamid et al. [30]. Another method recently used was the addition of nanoparticles to the base fluid to enhance the heat transfer rate. Nanosized particles mixed with the base fluid will effectively enhance the surface to volume ratio, improves the thermal conductivity of the mixture which causes an improvement in the hate transfer rate. Zafar Said et al. [31] used nanofluids to enhance the heat transfer rate from concentrated and non-concentrated solar collectors. Impact of nanoparticles on the fluid temperature difference, friction factor and pumping power were studied. Zawawi et al. [32] conducted experimental analysis using SiO2- TiO2 hybrid nanofluids to improve the performance of a multilayer solar collector.

After the thorough literature review on direct evaporative coolers, it is observed that various techniques or methods that have been used for performance enhancement may increase the energy consumption, water mass, or the related fixed and operating cost. As additional equipment is used, maintenance cost is also higher. In the different methods suggested, the exit air quality was poor which will be always a threat to the health of people. Further, in the prescribed methods in the literature, exit air relative humidity was also so huge which is also not favorable when the basic application is cooling. Hence, a scheme to cool the air to a much lower temperature without an appreciable increase in RH is required. Literature regarding the use of such methods is scanty. To overcome the above research gaps, an indirect–direct evaporative cooler has been devised. Steel plates are joined in such a way to build a cross flow heat exchanger between air and water which is termed as an indirect heat exchanger. Energy exchange between the two fluids is increased by blending aluminum oxide-based nanoparticles with water instead of only water. The output air from the indirect heat exchanger is made to flow through the celdek pads where it exchanges energy with water there by air gets humidified and its temperature reduces. Experiments have been conducted by varying the water flow rates, air velocities and volume fraction of nanoparticles. The results have been compared with various configurations such as only direct evaporative cooler, indirect–direct cooler without using the nanoparticles and are comprehensively discussed in the upcoming sections.

Methodology

Principle of indirect–direct evaporative cooler

Indirect cooler is one where the sensible heat transfer takes place between Al2O3-based nanofluid and air. Aluminum sheets are soldered together to shape them as a cross flow heat exchanger. Low temperature nanofluid is allowed to flow through the openings from the top whereas high temperature atmospheric air is entered in a perpendicular direction to it. Heat transfer takes place between the two fluids without mixing them together. Hence, a considerable drop in air DBT takes place without change in the absolute humidity. Exit air from the indirect heat exchanger is directed to the direct evaporative cooler wherein it interacts with water rushing through the packing material. Due to the vapor pressure difference between water and air, former gets evaporated by absorbing the latent heat of evaporation from air. It makes exit air humidified with lowering its temperature. Working principle of the indirect–direct evaporative cooler is shown in Fig. 1. In the psychrometric chart 1–2 represents sensible cooling air in the indirect heat exchanger and 2–3 shows the evaporative cooling that takes place with a celdek packing. Leaving air at state 3 will be of low temperature and humidified.

Fig. 1
figure 1

Working principle of an indirect–direct evaporative cooler

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Governing equations

Overall cooling effectiveness: Cooling effectiveness is the ratio of the temperature difference between entry and outlet DBT to the temperature difference between entry DBT and wet bulb temperature (WBT).

Cooling effectiveness of IDEC (indirect–direct evaporative cooler) is given by [9, 33]

$$\epsilon =\frac{{T}_{\rm di}- {T}_{\rm do} }{{T}_{\rm di }- {T}_{\rm wi}}$$
(1)

where Tdi are dry bulb temperature of air at inlet and outlet of IDEC (Indirect direct evaporative cooler) unit, and Twi is wet bulb temperature of air at inlet of evaporative cooler unit.

Cooling effect (Qc) The cooling effect gives the magnitude of energy removed from the air to reduce its temperature [9].

$$Qc={m}_{\rm a}\times {c}_{\rm p}\times \Delta T$$
(2)

where ma is mass flow rate of air (kgs−1), Cp is specific heat capacity (J kg−1 k−1) and ΔT is change in temperature at the inlet and outlet of the duct.

Considering the heat lost by the air is equal to the heat gained by the nanofluid Qc can also be written as [9]

$$Q_{\rm C} = m_{\rm a} \times \left( { h_{2} - h_{1} } \right) + ma \times cp \times \left( {T_{0} - T_{\rm di} } \right)$$
(3)

where h1 and h2 are enthalpy values at inlet and outlet of the direct cooler and T0 is the exit temperature of the air from the indirect heat exchanger.

Evaporation rate (mw): Evaporation rate is given by [9]

$$m_{\rm w} = m_{\rm a } \times \left( { w_{2 } - w_{1 } } \right)$$
(4)

where ma is mass flow rate of air, and w1 and w2 are specific humidity at inlet and outlet of the system.

Coefficient of performance (COP) COP is the ratio of total cooling effect to the work done by the system [9, 33].

$${\rm COP} = \frac{{Q_{\rm c} }}{{W_{\rm f} }}$$
(5)

where Qc is the cooling effect and Wf is the work done by the fan and the pump.

Different types of nanoparticles and nanofluids have been used for improving the heat transfer rate in various applications. They include nanoparticles of single metallic elements, their oxides, hybrid type, etc. [34]. Single metallic-based particles does not exhibit all the required and desired structural, thermal, hygroscopic and stability characteristics to achieve the specific application. Even though hybrid nanoparticles show better performance, their synthesis may be difficult and hence they are expensive. On the other hand, metallic oxide-based particles possess reduced size, improved biocompatibility, and higher specific area as compared to their pure metallic based. They can interact with cellular structures and can also exhibit antioxidant behavior which helps to improve their stability. Further in the present study, alumina-based nanoparticles have been chosen for the study. Due to its lower density, better solubility, better thermal conductivity, good chemical stability, aluminum oxide nanoparticles have been preferred over their counterparts. They are cost effective also easy to synthesize without having any difficulty in controlling the process parameters during synthesis and are also chemically stable with low hygroscopic [35]. Due to the above reasons, aluminum oxide nanoparticles have been selected in the present study.

Aluminum oxide nanoparticles with purity 99.99%, and particle size 20–30 nm were purchased from the nearby vendor and mixed with calculated quantity of water to form the nanofluid of required volume concentration. In order to mix the particles uniformly and to prevent the agglomeration of nanoparticles, it is subjected to ultra sonification in a sonicator mixed with a surfactant for 3 h. The prepared nanofluid is visually inspected every day to see any agglomeration of nanoparticles. After continuous observation of two weeks, it is seen that there was no sedimentation of nanoparticles at the bottom the container. Hence, it was concluded that there was not any agglomeration of the alumina nanoparticles, and hence, it was considered to be of single phase and used in the indirect–direct evaporative coolers. The basic properties of the prepared nanofluid with the varied volume fractions could be calculated by the following equations [36].

$${\rm dynamic viscosity of Nanofluid},\mu_{\rm nf} = \left( {1 + 2.5\phi } \right)\mu_{\rm f} \left( {\phi \le 5\% } \right)$$
(6)

where \({\mu }_{\rm nf}\) and \({\mu }_{\rm f}\) are the specific heat values of the nanofluid and base fluid, respectively, and \(\phi\) is the volume fraction of the nanoparticle [36].

$${\rm Density of Nanofluid},\rho_{\rm nf} = \phi \rho_{\rm s } + \left( {1 - \phi } \right)\rho_{\rm f}$$
(7)

with ρnf, ρs and ρs are the density of nanofluid, solid and base fluid, respectively [36]

$${\text{Specific }}\,{\text{heat}}\,{\text{ of}}\,{\text{ the}}\,{\text{ nanofluid}}\, \, C_{{{\text{pnf}}}} = \frac{{\phi \rho_{\rm s} C_{\rm ps} + \left( {1 - \phi } \right)\rho_{\rm f} C_{\rm pf} }}{{\rho_{\rm nf} }}$$
(8)

where Cpf and Cps are the specific heat of solid and base fluid, respectively.

\(Equivalent Thermal conductivity of Nanofluid,\) [36]

$$K_{\rm nf} = \frac{{k_{\rm s} + 2k_{\rm f} + 2\left( {k_{\rm s} - k_{\rm f} } \right)\left( {\left( {1 + \beta } \right)^{3} } \right)\phi \left( {k_{\rm s} - k_{\rm f} } \right)}}{{k_{\rm s} + 2k_{\rm f} - 6.166\phi \left( {k_{\rm s} - k_{\rm f} } \right)}}k_{\rm f}$$
(9)

where \({k}_{\rm s}\, \text{and} \,{k}_{\rm f}\) are the thermal conductivity of solid nanoparticle and base fluid, respectively, and \(\phi\) being the volume fraction. β is the ratio of nanolayer thickness to the original particle radius and taken as 0.1.

Experimental set up and procedure

Construction and working.

Aluminum sheets with dimension 250 mm × 250 mm × 0.5 mm thick are joined together by maintaining a gap of 10 mm between them. Air is able to move through these plates only horizontally as the top ends being closed. This is in contact with another 2 adjacent plates which are also maintained with a gap of 10 mm. These plates are constructed in such a way that are open at both ends so that nanofluid flows through the plates and is being collected at the tank kept below. Likewise, 24 passages have been created which fulfil the necessary area requirements of the indirect heat exchanger. The direct contact between nanofluid and air is not allowed, instead only sensible energy transfer takes place between them.

A cellulose pad with dimension of 250 mm*250 mm *70 mm is inserted inside the duct made of plywood with the dimensions 270 mm * 270 mm and of length 1500 mm. A fan with a capacity of 90W is used for blowing the air in the duct. To vary the air flow rates, a dimmerstat is used. Water and nanofluid to be supplied to the system are stored in separate tanks. Tanks acts as an infinite reservoir, where constant head is maintained to avoid pressure drop at the flow side. Two pumps are used for sending water back to the overhead tank. From tank, water is made to flow through rotameters used for measuring the flow rate of nanofluid and pure water separately. To get the spraying effect at inlet to both types of coolers, plates are provided with holes which in turn distribute water and nanofluid uniformly along the entire surface. Water coming out of the system is collected in a tank. The schematic sketch is represented by Fig. 2. DBT, WBT at the various points such as duct entry, immediately at the exit of IEC, entry to the DEC and exit of the duct are measured by using thermocouples inserted. A hygrometer is used for measuring the RH at the respective locations. An anemometer is used to measure the air velocities at the respective points. Table 1 gives the details of the measuring instruments and their specifications.

Fig. 2
figure 2

Schematic representation of an indirect–direct evaporative cooler

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Table 1 Instrument specification
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Materials and method

Al2O3 nanoparticles with the required mass to obtain the specified volume fraction are mixed with water and subjected to ultra sonification to avoid agglomeration of nanoparticles in the solution. Nanofluid is kept idle for a week and visual observation has been done to test for any deposition or agglomeration of particles at the bottom. It was found that no suspension or deposition was found at the week end. Hence, it was presumed that prepared nanofluid was free from agglomeration. The prepared nanofluid solution is used as a cooling agent in the indirect heat exchanger. Ordinary water is used for direct evaporative cooler. The experiments have been conducted in the test rig discussed in the previous section by varying the operating variables as shown in Table 2.

Table 2 Experimental Matrix
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Results and discussion

Temperature difference (ΔDBT)

ΔDBT in the indirect heat exchanger.

To the indirect cooler, initially water is admitted which cools the inlet air without any change in the specific humidity. Nanoparticle volume fraction is varied from 0.1 to 2.5% and the temperature difference between the entry and the exit points are evaluated and shown in Fig. 1. Figure also depicts the temperature difference when there are no nanoparticles used. When the water flow rate is increased for any definite air flow rate initially ΔDBT increases, reaches maximum at a flow rate of 3 LPM, further increase in the flow rate ΔDBT drops. This may be due to the reduced energy exchange between the water and air both in direct and indirect coolers due to the excess velocity of water. Similar trend has also been noticed with considering the air flow rate variation. In the tested conditions, it is seen that 5 ms−1 observed maximum ΔDBT. Al2O3 Nanoparticles have higher thermal conductivity and when blended with water, effective thermal conductivity of the mixture is higher than pure water. This allows a higher heat transfer rate between water and air in the indirect heat exchanger. As the two fluids are unmixed with each other, it involves only sensible heat transfer between air and water causing reduction in air temperature without change in absolute humidity. Further, increase in nanoparticle volume fraction, shows an increasing–decreasing trend for ΔDBT. Initially, as the volume fraction is increased, a gradual increase in heat transfer takes place as observed in higher Δ DBT values. When the particle concentration is increased up to 2.5%, it is observed that ΔDBT dropped. As the nanoparticle concentration in base fluid is in excess, effective viscosity of the mixture increases which offers resistance to the water flow. Nanoparticles may also agglomerate locally and may reduce the thermal conductivity of the total nanofluid. All these characteristics will lead to poor heat transfer rate by which ΔDBT showed a decreasing trend as seen clearly in Fig. 3. When the nanoparticle concentration is increased from 0 to 2.5%, ΔDBT increased by 9% for 3 LPM and maximum air flow rate tested. Error involved during the measurements have been reported as error bars in Fig. 3. It is seen that the average error is within 0.9 − 2.2% for the tested cases.

Fig. 3
figure 3

Temperature difference between entry and exit of indirect heat exchanger

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ΔDBT between the inlet and exit of the indirect–direct evaporative cooler

In the indirect heat exchanger, air temperature is reduced by the indirect contact between air and water. Air then enters the direct heat exchanger where it interacts with the water flowing in the packing material. As a result, water evaporates by absorbing the latent energy from air and hence air temperature is further reduced. Figure 4 shows the total drop in air temperature between its entry and exit points considering both direct and indirect heat evaporative cooler for various air velocities and water flow rates and also nanofluid with varying concentrations.

Fig. 4
figure 4

Temperature difference between entry and exit of indirect–direct heat exchanger

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For all tested conditions, initially at lower air flow rates, ΔDBT is lower and found to be gradually increased. When the flow rate exceeds 5 ms−1, ΔDBT dropped. When the flow rate is higher, interaction between air and water will be reduced both in indirect and direct heat exchangers, which reduces ΔDBT in total. The results are similar to that obtained by authors in the published literature [37]. On comparing the total temperature drop, for different cases, nanoparticle added case gives higher ΔDBT than the normal water. Efficiency of the indirect heat exchanger will be improved by adding the nanoparticles to the water so that it aids in the indirect heat transfer rate. Increased surface to volume ratio, enhancement in the effective thermal conductivity due to the increased Brownian movement of nanoparticles in the mixture contributes to the enhanced heat transfer rate. Hence, ΔDBT in the indirect heat exchanger will be higher. When further this air is allowed to flow through the celdek packing, further drop occurs by the evaporation of water. This cumulatively added to the ΔDBT and hence a large drop in the exit temperature is noticed. On the contrary, when it exchanges heat without nanoparticles, there is a moderate drop in the temperature in the indirect and further cumulatively added with the temperature drop in the direct cooler, but the resultant or total ΔDBT is found to be less than the former case. From Fig. 4, it is seen that when the water flow rate is maintained at 5 LPM, best results have been observed in terms of maximum drop in DBT values. Higher flow rates will reduce the retention time of water in both indirect heat exchanger and direct evaporative cooler, resulting in reduced energy transfer between the fluids and hence lower ΔDBT.

Difference in the RH

ΔRH in direct evaporative cooler

Figure 5. shows the RH variation with the air velocity between the inlet and exit of the evaporative cooler for only direct type. It shows the increasing trend of RH at the exit with air velocity till 5 ms−1. After this, contact time between air and water in the celdek packing reduces which hinders the evaporation rate and hence lower increase in RH values are observed at the exit or in other words ΔRH drops. The results are in line with the reported literature values [33]. Similar trend has been observed by varying the water flow rates. At lower flow rates, the minimum quantity of water interacts with air and increases as the flow rate is increased. It reaches an optimum value where evaporative rate is found to be maximum and drops thereafter. This leads to again drop in ΔRH.

Fig. 5
figure 5

Δ RH between entry and exit of duct in a direct heat exchanger

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ΔRH in indirect–direct evaporative cooler

In the indirect–direct type of cooler, initially air will be circulated in between the plates where the nanofluid flows in the adjacent plates. Figure 6 shows the ΔRH variation when the nanoparticle volume fraction varied from 0 to 2.5%. Due to higher thermal conductivity of the nanofluid, heat transfer capacity of water increases by air will be cooled to a much lower temperature. As the heat exchange process is sensible heat transfer, exit temperature of air will be reduced by huge values without appreciable change in specific humidity. Even though the exit specific humidity is same, RH slightly increases due the reduced air temperature at the exit of the indirect heat exchanger. Later air will be directed to the direct heat exchanger, wherein it interacts with the normal water in the celdek packing and by evaporative cooling, air temperature will be reduced with increased RH. But the rise in humidity is smaller as compared to the only direct evaporative cooler from Figs. 5 and 6 which is purpose of the present study. Further, for any fixed air flow rate and water flow rate, as the nanoparticle volume fraction is increased, temperature drop in the indirect heat exchanger is increased. Hence entry air to the direct cooler is at a lower temperature and slightly higher RH. This condition reduces the humidification capacity of air as compared to the condition of only direct type. It leads to a lower evaporation rate or lower RH at the exit. Hence, ΔRH will be lower at the exit. Since the temperature has already been lowered in the indirect exchanger, cumulative temperature drop from the indirect–direct type of cooler is higher, with increase in RH being lower. As the nanoparticle volume fraction is increased temperature drop in the indirect heat exchanger is higher as seen in the previous section which further reduces Δ RH between entry and exit. When the volume concentration is above 1%, temperature decrement in the indirect heat exchanger reduces. It may due to the reduced thermal conductivity and increased viscosity of the nanofluid. Agglomeration of the nanoparticles may take place resulting in reduced Brownian motion of the particles in water. Once the exit temperature in the indirect heat exchanger is reduced, it is seen that ΔRH at the duct exit is found to be higher indicating higher evaporation rate. Hence, an optimum volume fraction of nanoparticles is needed to enhance the performance of an indirect–direct evaporative cooler. From the studies, it is realized that when the nanoparticle concentration is increased from 0 to 2.5%, exit RH reduced by 9.4% for best flow rate condition of 3 LPM. From the error graph, it is seen that the average error between the various experimental trials conducted is between 0.5 to 1.8%

Fig. 6
figure 6

Δ RH between entry and exit of duct in an indirect–direct heat exchanger

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Cooling efficiency

Cooling efficiency of indirect–direct evaporative cooler without nanofluid.

Cooling efficiency is the ratio of actual drop in the temperature to the maximum possible temperature drop considering the WBT of air. Figure 7 shows the cooling efficiency variation for the indirect–direct humidifier without the application of the nano fluid for different air and water flow rates. At lower flow rates, cooling efficiency is lower due to the lower ΔDBT values especially in the indirect heat exchangers. It gradually increased as the air velocity and water flow rates increased, which is similar to the nature obtained by the authors in the published literature [33, 38]. When both flow rates are increased, contact between the two fluids in both the indirect and direct coolers will be better and hence a higher drop in DBT is expected in both types which cumulatively is higher as compared to that of lower flow rates. At too high air velocity and water flow rates, both the fluids will cover the entire available heat transfer area so fast that energy exchange will be reduced. This decreases the ΔDBT values and leads to a lower the cooling efficiency.

Fig. 7
figure 7

Cooling efficiency of indirect–direct evaporative cooler without nanofluid

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Cooling efficiency of indirect–direct evaporative cooler with Al2O3 nanofluid

Figure 8. shows the variation of cooling efficiency with varying water flow rate and nanoparticle concentration considering the nanofluid instead of only water in the indirect heat exchanger measured for 5 ms−1 velocity of air flow. For lower volume fractions, mass of the nanoparticles will be less. This reduces the effective surface area and also the Brownian movement of the particles in water. These two characteristics lead to lower the effective thermal conductivity of the mixture. When the nanoparticle volume fraction is increased, effective thermal conductivity of the mixture improves. It leads to a better heat transfer from the air to water, therefore air temperature will be reduced to a lower value than without nanoparticles. Hence, exit temperature will be reduced to a higher value with minimum increase in exit RH. These results will further influence its action on the direct evaporative cooler that results in a huge drop of air temperature with moderate rise in RH. As discussed in Sect. 3.1.1 and 3.1.2, it is seen that temperature drop in this case is comparatively higher than without nanofluid. This directly influences the efficiency of the cooler. As seen in Fig. 8, the cooling efficiency values for the present case is higher than the values found in Fig. 7 illustrating the advantage of using Al2O3 nanoparticles with water. As observed in the previous section, similar trend has been noticed considering the variation with water flow rate, with 3 LPM showing the best. Nanoparticle volume fraction of 1% showed the best results. Increasing the volume fraction deteriorates the cooler performance with showing negative trend for temperature reduction in the indirect heat exchanger. Error analysis indicates that the error involved during the experimentation is minimum and for the cooling efficiency values it lies between 0.8–2.1%.

Fig. 8
figure 8

Cooling efficiency of indirect–direct evaporative cooler with nanofluid

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Energy consumption

Energy required to blow the air from the inlet to the exit of the duct has been recorded and shown in Fig. 9. Energy is being consumed by the blower to blow the air whereas pump is used in the indirect heat exchanger to allow the liquid to flow through the plates. Energy consumed by the pump for all cases is equal, as the flow rate is being controlled by the valves. In addition, they also constitute a small value related to the energy consumed by the blower. It is clearly noted that energy consumption increases as the air velocity increases. Higher energy is required to push air through the cooler by overcoming the frictional losses in order to maintain the velocity that is needed to have a circulation inside the room. When the air velocity is increased from 3 to 6 ms−1, energy consumption increases by 49%. The results are analogous with the results published in the literature [33]. Maximum error values of 7.5% were noticed during the experimental analysis.

Fig. 9
figure 9

Variation of energy consumed with air flow rate

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Coefficient of performance (COP)

COP of indirect direct evaporative cooler without nanofluid.

COP is one of the important assessments in an evaporative cooler as it gives the measure of energy efficiency of the system. Figure 10 shows the COP values for the system without using the nanofluid. When the water is pushed through the indirect heat exchanger, due to its limited heat capacity, heat transfer rate will be minimum and that results in a moderate ΔDBT. As the air velocity is increased, both cooling effect and energy consumption increases. Rate of rise in the cooling effect surpasses the increase in energy consumption that results in increase of COP. From the previous sections, it is seen that when the air velocity increases above 5 ms−1, cooling effect decreases while energy consumption increases. This leads to a drop in COP value. A similar trend is also noticed by the variation in water flow rate with 3 LPM showing the best results.

Fig. 10
figure 10

COP of indirect–direct evaporative cooler without nanofluid

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COP of indirect–direct evaporative cooler with the nanofluid.

When the nanoparticles are used, the ΔDBT values are found to be higher than the case without nanoparticles especially in an indirect heat exchanger. This again aids the heat transfer rate in the direct exchanger and hence cumulative ΔDBT and heat transfer rate will be higher whereas input energy remaining almost same for all the cases. Hence, COP will increase eventually. This is well depicted in Fig. 11. With the addition of nanoparticles also, maximum COP is noticed for 5 ms−1 of air velocity. Both lowering or increasing air velocity leads to lower COP values. The reason being the reduced heat transfer rate as discussed in the Sect. 3.2.1.

Fig. 11
figure 11

COP of indirect–direct evaporative cooler with nanofluid

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Variation of COP with the addition of nanoparticles and error values obtained is shown in Fig. 11. From the graph, it is seen that the average error values are within 1.4—2.7%. For any fixed air velocity, as the concentration of nanoparticles has been increased, COP also increased. Addition of Al2O3 nanoparticles will improve ΔDBT values, in turn will increase the heat transfer with the energy consumed remaining same as noticed from the previous sections. This leads to an increase in COP values. But when the nanoparticle concentration exceeded 1%, it shows the negative influence on the ΔDBT, thereby reducing the heat transfer rate and hence lower COP values. It clearly demonstrates that an optimum combination of air velocity, water flow rate and nanoparticle combination is essential to operate the indirect–direct evaporative cooler at a higher COP.

Comparison of various configurations of the evaporative Coolers.

To clearly understand the performance of the evaporative cooler, 3 different configurations namely direct, indirect–direct and indirect–direct with nanofluids have been compared. Figure 12 shows the comparison of the configurations for different parameters tested. In the direct type, water directly enters the evaporative cooling pads. Energy transfer takes place between water and air in the pads there by air gets humidified with temperature being reduced. Figure shows the temperature variation for three different cases. In case of direct cooler, ΔDBT will be the lowest whereas indirect–direct with nanofluids is the highest. Hence, exit air will be having a very low temperature. In the direct type of cooler, ΔRH will be higher due to the higher evaporation rate. It is lowered in the indirect heat exchanger and is further lowered when the bare water is replaced by the nanofluid. Humidity from the exit of the direct type is so high and is not ideal for human beings as it causes the growth of microbes at higher humidities. Nanofluids having a higher heat capacity will be able to receive more energy so that air is being cooled to a higher value in the indirect heat exchanger without change in specific humidity of air. Further, when the air is taken to the direct type, further reduction in temperature takes place with reasonable rise in the RH which is a favorable condition for human beings. In summary, the temperature at the exit is decreased by 13.1% and 39% when direct evaporator cooler is made as indirect and indirect–direct with nanofluids. ΔRH values correspondingly decrease by 14% and 27% as compared to the former case.

Fig. 12
figure 12

Comparison of various configurations of evaporative coolers

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Similar observations have been drawn on comparing the cooling efficiency values, with indirect–direct with nanofluids showing the highest. When the nanofluid is used for cooling in the indirect heat exchanger it cools the air by a huge value which increases the total ΔDBT between entry and exit. Hence, efficiency will be maximum in this case. It is higher by 14.2% and 21% as compared with the 2 former cases. Similar results have been noticed with COP values as the cooling effect will be maximum when the nanofluid is used. The values are correspondingly 15.6% and 18% higher than the values for indirect–direct and only direct type, respectively. The above values clearly depict the advantage of adding Al2O3 nanoparticles to the water and using them in a combination of indirect–direct type of evaporative coolers to have an enhanced cooling with minimum rise in air humidity values.

Comparison with literature values.

Present work results have been compared with the similar type of work conducted by the various researchers in both only direct and indirect–direct type of evaporative coolers. As the methodology used, operating conditions, design conditions were exactly not the same as that of the present work, trend or the nature of variation of the important parameters have been compared. It is seen that the present work comprising of the indirect–direct combination of evaporative cooler with using the alumina-based nanoparticles gave comparable results with the work conducted by other researchers. The ΔRH values have seem to lower as compared to the direct types of coolers with attaining high cooling efficiency and COP (Table 3).

Table3 Comparison with various types of evaporative coolers
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Conclusions

In the present work, an effort has been taken to increase the performance of an evaporative cooler by using an indirect heat exchanger to reduce the DBT of air using alumina-based nanofluids and a direct evaporative cooler to further cool it by using evaporation principle. Experiments have been conducted by varying the air flow rate, water flow rate and also alumina nanoparticle volume concentrations. The conclusions obtained from the study can be summarized as follows:

  1. 1.

    Addition of nanoparticles have shown an overall increase in the performance of the cooler in terms of drop in temperature and relative humidity.

  2. 2.

    By adding 1% Al2O3 nanoparticles, exit temperature of air reduced by 13.1% and 39.2% as compared to the case without nanoparticles and only direct evaporative coolers, respectively.

  3. 3.

    For the same condition, exit RH reduced by 27.2% as compared with only direct evaporative cooler.

  4. 4.

    Performance of the cooler increased with increasing the water flow rate. For the nanoparticle added condition, when the flow rate is increased from 1 to 3 LPM, ΔDBT, humidification efficiency and COP increased by 24%, 23.3% and 62%, respectively. Upon increasing the flow rate to 4 LPM, the above parameters reduced by 8%, 6.5% and 8.4%, respectively.

  5. 5.

    3 LPM showed the best performance with highest humidification efficiency and COP of 96% and 5.9, respectively.

  6. 6.

    Tested results showed an increase in the cooler performance in terms of temperature drop decrease in RH, cooling efficacy and COP as the air velocity is increased, with 5 ms−1 showing the maximum values. The corresponding values are 13.1%, 27,2%, 14.2% and 15.6%, respectively, higher for nanofluid case as compared to the configuration without nanoparticles.

  7. 7.

    Performance of the cooler is improved by increasing the nanoparticle concentration from 0 up to 1% and when further increased to 2.5%, performance deteriorated in terms of reduced ΔDBT, increased ΔRH, drop in cooling efficiency, and drop in COP by 4%, 4.3%, 2.3% and 5.1%, respectively, for air velocity of 5 ms−1 and the water flow rate of 3 LPM.

  8. 8.

    For a definite water flow rate, as the air velocity is increased, energy consumption was found to augmented. When the air velocity is increased from 3 to 6 ms−1, energy consumption increases by 49%.

  9. 9.

    Exit temperature decreased by13.1% and 39%, when the direct evaporative cooler is changed to as indirect–direct and indirect–direct with nanofluid, respectively. For the above combinations, exit RH is reduced by14% and 27%, respectively.

Hence, it is concluded that an evaporative cooler performance can be improved substantially by using a combination of indirect–direct methods and also by using nanoparticles. It is found to be very effective when the goal is only to cool the air by restricting the humidity rise. Since the exit RH is under control, the risk of high humidity can be minimized which may lead to a good thermal comfort conditions. Hence, this novel technique is sustainable, energy efficient as compared to conventional techniques, pollution free and also environmental friendly.

Limitations of the present study and future scope.

Present evaporative cooler is most efficient in dry and non-humid climates and as the outside humidity increases, system efficiency reduces. Hence, it may not be efficient and not suitable for high humid climates.

Instead of using commercially available celdek packings, various biomass-based packings could be replaced. Similarly studies can be extended by using nanoparticles of different origins their blends and evaporative cooler performance can be studied.