Optical density and velocity measurements in cryogenic gas flows
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- Jensen, O.S., Kunsch, J.P. & Rösgen, T. Exp Fluids (2005) 39: 48. doi:10.1007/s00348-005-0966-8
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This paper presents the application of optical measurement techniques in dense-gas flows in a heavy-gas channel to determine planar two-component (2C) velocity profiles and two-dimensional (2D) temperature profiles. The experimental approach is rather new in this area, and represents progress compared with the traditional techniques based on thermocouple measurements. The dense-gas flows are generated by the evaporation of liquid nitrogen. The optical measurement of both the velocity and density profiles is accomplished by the implementation of particle image velocimetry (PIV) and background-oriented schlieren (BOS) systems. Supplemental thermocouple measurements are used as independent calibrations to derive temperatures from the density data measured with the BOS system. The results obtained with both systems are used to quantify the dilution behavior of the propagating cloud through a global entrainment parameter β. Its value agrees well with the results obtained by earlier studies.
The widespread storage and transport of liquefied gases like propane can result in the accidental release into the environment of dense-gas clouds. Therefore, the spreading and dilution behavior of dense gases is always a major concern in current risk assessment studies. The spreading behavior of gases strongly depends on the density ratio of the gas to the ambient air. Due to the low temperature of the accidentally released clouds, their density is higher than the ambient value and their propagation dynamics depends on gravity. However, with a density sufficiently close to the one of the ambient air, the gas behaves passively, i.e., it moves with the prevailing flow conditions of the surrounding air. The spreading and dilution behavior of these so-called heavy-gas clouds is investigated here with particle image velocimetry (PIV) (Raffel et al. 1998; Scarano 2002; Willert and Gharib 1991). Additionally, measurements are carried out with the background-oriented schlieren (BOS) method (Dalziel et al. 2000; Meier 2002; Richard and Raffel 2001), which are supplemented by separate thermocouple reference measurements for the extraction of 2D temperature data. The purpose of this study is to assess the applicability of the present measurement techniques in cryogenic gas clouds and to support the modeling activities.
2 Experimental setup
Currently, two different release scenarios can be used to generate cryogenic heavy-gas clouds with variable properties. In the first scenario, the evaporated liquid nitrogen is suddenly released from the release chamber. With the second scenario, it is possible to continuously release a cryogenic gas cloud from a heated Dewar container filled with liquid nitrogen. The additional heating required in this case is provided by immersion heaters.
The continuous-release setup represents a generic configuration, allowing the extraction of well defined parameters of the flow. An electrical heating system is installed in a reservoir of liquid nitrogen large enough to create steady-state measuring conditions for the experiments. By varying the applied power, the amount of evaporated nitrogen can be varied continuously.
3 Background-oriented schlieren measurements
The results presented here are, as stated before, averaged along the line-of-sight. This approach is best used with 2D flows. The correlation of measurements from thermocouples placed 66 cm apart at the same distance downstream from the source of the flow show correlation values of up to 0.8. This justifies the validity of the setup for the measurements presented in this section.
4 Particle image velocimetry measurements
An objective of the present experiments was not only to generate a better insight into the basic physics of a dense-gas cloud, but also to extract and interpret the relevant parameters supporting the modeling activities. An important issue is the dilution behavior of a cryogenic dense-gas cloud, which can be described globally by an entrainment parameter.
The temperature measurements yield almost constant temperatures over the height of the cloud, as documented by the temperature profiles in Fig. 15. The homogeneous temperature distribution is an indication for the intense mixing of the cloud and the dependence of β on heat transfer effects (Kunsch and Fanneløp 1995).
The definition of the entrainment parameter presented so far is simple, and several references making use of β are available for comparison and interpretation (Fanneløp 1994; Ruff et al. 1988). In addition, risk assessment studies, including heavy-gas dispersion in the chain of events, call for simple tools allowing rapid estimates of the propagation and dilution behavior. So, the shallow-layer models for dense-gas clouds require parameters accounting for the complex flow phenomena governing the frontal dynamics or the dilution behavior in a global manner. Hence, even nowadays, there are still good reasons for concentrating the information on the entrainment and dilution behavior of the present experiments in a single parameter. The easy way for extracting the corresponding information, as demonstrated in the present study, may be considered as an example. However, the optical methods presently used are suitable for a more detailed insight into the structure of a dense-gas cloud, e.g., regarding the frontal dynamics or the mixing zone on top of the dense-gas layer.
6 Summary and conclusions
The main goal of this study was to investigate the propagation of cryogenic dense-gas clouds with optical methods. Therefore, a particle image velocimetry (PIV) system and a background-oriented schlieren (BOS) system were set up to determine the two important properties; velocity and temperature.
The velocities presented are computed from the flow images obtained using PIV. Due to the properties of the “natural” seeding used, only the textures in the images can be correlated; no individual particles can be identified. As a benefit, this makes it possible to process comparatively large fields of view. Averaged planar two-component (2C) velocity profiles in a viewing area with a size of 40×40 cm could be computed for different positions in the facility.
The temperatures presented in 2D are measured with the BOS system. With a correlation algorithm, the apparent displacement of the image due to variations in the refractive index can be calculated. An in-situ calibration was performed based on discrete calibration temperatures acquired with thermocouples.
The modeling activities could be furthered by using the combined experimental results for velocity and temperature to extract a global parameter that quantifies the dilution of the cryogenic gas cloud during its propagation. An entrainment coefficient β can be derived from a mass balance and a horizontal momentum balance for a differential “control volume” in the heavy-gas channel. The coefficient is close to the values obtained in similar studies conducted by Ruff et al. (1988).
The results presented in this study show the general feasibility to investigate the spreading of cryogenic dense-gas clouds with optical methods. With the applied techniques, the two important distributions of velocity (planar 2C) and temperature (2D) of the propagating clouds could be measured. A PIV system and a BOS system were designed for this purpose. In the context of optical velocity measurements, the simultaneous visualization of the cryogenic gas flow and the ambient air was realized successfully, due to the large field of view employed. The velocimetry system and the schlieren system both allow to derive two-dimensional data. Small-scale structures in both the velocity and temperature, were, however not accessible with the optical methods chosen.