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Numerical heat and mass transfer analysis of a cross-flow indirect evaporative cooler with plates and flat tubes

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Abstract

In this study the performance of an indirect evaporative cooling system (IECS) of cross-flow configuration is numerically investigated. Considering the variation of water film temperature along the flowing path and the wettability of the wet channel, a two-dimensional theoretical model is developed to comprehensively describe the heat and mass transfer process involved in the system. After comparing the simulation results with available experimental data from literature, the deviation within ±5 % proves the accuracy and reliability of the proposed mathematical model. The simulation results of the plate type IECS indicate that the important parameters, such as dimension of plates, air properties, and surface wettability play a great effect on the cooling performance. The investigation of flow pattern shows that cross-flow configuration of primary air with counter-flow of secondary air and water film has a better cooling performance than that of the parallel-flow pattern. Furthermore, the performance of a novel flat tube working as the separating medium is numerically investigated. Simulation results for this novel geometry indicate that the tube number, tube long axis and short axis length as well as tube length remarkably affect its cooling performance.

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Abbreviations

a :

Tube long axis length (m)

b :

Tube short axis length (m)

c p :

Specific heat capacity at constant pressure (J/kg K)

dx :

Dimension of control element along x-axis (m)

dy :

Dimension of control element along y-axis (m)

D dh :

Degree of humidity (g/kg)

D eq :

Equivalent diameter (m)

f :

Friction factor

h :

Convective heat transfer coefficient (W/m2 K)

h d :

Convective mass transfer coefficient (kg/m2 s)

g :

Acceleration due to gravity (m/s2)

i :

Specific enthalpy (J/kg)

i o :

Specific enthalpy of vapor at 0 °C (J/kg)

k:

Thermal conductivity (W/m2 K)

L x :

System length (m)

L y :

System height (m)

Le:

Lewis factor

m :

Mass flow rate (kg/s)

N :

Number of plates or tubes

N t :

Number of tubes in one column

N l :

Number of tubes in one row

Nu :

Nusselt number

Pr :

Prandtl number

ΔP :

Pressure drop (Pa)

Q :

Heat transfer rate (W)

Re :

Reynolds number

r :

Relative pitch

S :

Tube spacing (m)

T :

Temperature (°C)

U :

Overall heat transfer coefficient (W/m2 K)

ν :

Velocity (m/s)

z :

Channel width (m)

ρ :

Density (kg/m3)

δ :

Thickness (m)

σ :

Surface wetting factor

μ :

Dynamic viscosity [kg/(m s)]

ε :

Efficiency

Γ:

Water linear flow rate [kg/(m s)]

ω :

Humidity ratio (g/kg dry air)

ϕ :

Relative humidity

a :

Dry air

c :

Calculated results

e :

Experimental data

i :

Inlet

int :

Water and secondary air interface

l :

Longitudinal

lam :

Laminar flow

o :

Outlet

p :

Primary air

ps :

Primary air to secondary air

pw :

Primary air to water

s :

Secondary air

se :

Sensible

la :

Latent heat

m :

Moist air

sat :

Saturated/saturation

t :

Transversal

tur :

Turbulent flow

ν :

Vapor

w :

Water film

wb :

Wet bulb

x :

x axis direction

y :

y axis direction

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Acknowledgement

The authors gratefully acknowledge the kind support of National Research Foundation CRP Programme NRF2011NRF_CRP003_003 funding for this research.

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Authors and Affiliations

  1. Department of Mechanical Engineering, National University of Singapore, 9, Engineering Drive 1, Singapore, 117576, Singapore

    K. J. Chua, J. Xu, X. Cui & K. C. Ng

  2. Engineering Science Programme, National University of Singapore, 9, Engineering Drive 1, Singapore, 117575, Singapore

    K. J. Chua & M. R. Islam

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  1. K. J. Chua
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  2. J. Xu
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  3. X. Cui
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  4. K. C. Ng
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Correspondence to K. J. Chua.

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Chua, K.J., Xu, J., Cui, X. et al. Numerical heat and mass transfer analysis of a cross-flow indirect evaporative cooler with plates and flat tubes. Heat Mass Transfer 52, 1765–1777 (2016). https://doi.org/10.1007/s00231-015-1696-y

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  • DOI: https://doi.org/10.1007/s00231-015-1696-y

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