(34h) Experimental Study of the Contact Line Region for a Pure Fluid and Binary Fluid Mixture on the ISS

The Constrained Vapor Bubble (CVB) is a wickless heat pipe that is designed to produce a simple, light, and reliable heat transfer system that can be used for cooling critical components of spacecraft. A full-scale fluid experiment flown on the International Space Station (ISS) studied the thermophysical principles underlying change-of-phase heat transfer systems. The CVB system consists of a relatively simple setup - a quartz cuvette with sharp corners partially filled with either pentane (CVB1) or an ideal mixture of pentane and isohexane (CVB2) as the working fluids. The Light Microscopy Module (LMM) was used to determine the two-dimensional thickness profile of the menisci formed at the corners of the cuvette, along with temperature and pressure measurements. Interfacial forces dominate in these extremely small Bond number systems. The transport processes were found to be complex despite being conceptually simple. At the heated end of the CVB, due to a high temperature gradient, we observed â??Marangoni flowâ? in the direction of increasing surface tension toward the cooler end when pure pentane was used as the working fluid. This temperature induced Marangoni flow prevents liquid from recirculating to the heater end, and therefore reduces the effectiveness of the heat pipe. Recently, several research groups used a water and alcohol mixture, with a low concentration of alcohol, resulting in better performance of the heat pipe [1, 2]. The alcohol/water combinations were peculiar in that for a certain composition range, the surface tension increases with increasing temperature thereby driving liquid toward the hotter end. It was believed that changing the direction of the Marangoni stress or reducing its magnitude by differential evaporation of an ideal binary mixture would also improve the performance of the heat pipe. In our experiment, in order to mitigate Marangoni flow, we use a mixture of 94 vol%-pentane and 6 vol%-isohexane. This mixture forms an ideal liquid containing a low concentration of lower vapor-pressure and higher-interfacial-tension, isohexane. With isohexane added, the composition of the liquid will change with position in the heat pipe. The normal change in interfacial tension due to the temperature gradient will be opposed by an opposite change in interfacial tension due to the change in mixture composition.

On the large scale, to study the heat transport phenomena in the CVB heat pipe, we developed a one-dimensional heat transfer model which was validated using data from the ISS experiments. Based on this model, the internal heat transfer coefficient in the mixture case was improved to almost twice that of the pure pentane case. We also found that the Marangoni stress reduced by about five times [3].

On the small scale, the junction of the liquid with the vapor and the solid substrate is of great interest. This junction has been called the contact line region. The concept of phase change heat and mass transfer in the three-phase contact line region and the dynamics of how that contact line behaves are essential to understanding and controlling the wide variety of processes including boiling, evaporation, condensation, and coating processes. The CVB module is an ideal experimental setup for studying complex fluid flow, evaporative heat transfer, and interfacial phenomena at multiple scales. Using interferometry technique and high resolution (50X) images from the LLM, we were able to measure the two-dimensional thickness profile of the menisci formed at the corners of the cuvette. This data allowed us to obtain, with high accuracy, the curvature gradient at the liquid-vapor interface which is one of the main parameters that controls fluid flow toward the contact line region. Our results showed that the curvature gradient is a strong function of local heat flux and concentration. For both the pure pentane and the mixture cases, the curvature gradient increases with increasing local heat flux in the evaporation region, and approaches constant value moving toward the condensation region. For the mixture case, the curvature gradient increases with decreasing pentane concentration. Comparing the pure pentane and the mixture cases at the same local heat flux, we found that the curvature gradient is larger for the mixture case (up to five times larger); and the difference between the two cases increases as pentane concentration decreases. Our data also showed that the curvature gradient profile along the evaporation region of the heat pipe is divided into two regions: the region from the heater end to â??the central dropâ? and the region from the central drop to the end of the evaporation section. The division location of the two regions corresponds to the location of maximum internal heat transfer rate. We also saw the change in liquid film distribution in the region from the heater end to the central drop before and after the maximum point is observed in the internal heat transfer rate profile. Before the maximum point is observed, the thin liquid film moves away from the wall as we move toward the heater; after the maximum point is observed, it moves toward the wall.

Supported by NASA under Grant number NNX13AQ78G.

References

1. N. di Francescantonio, R. Savino, Y. Abe, New alcohol solutions for heat pipes: Marangoni effect and heat transfer enhancement, Int. J. Heat Mass Transfer 51 (2008) 6199â??6207.

2. K.M. Armijo, V.P. Carey, An experimental study of heat pipe performance using binary mixture fluids that exhibit strong concentration Marangoni effects, J. Therm. Sci. Eng. Appl. 3 (3) (2011) 031003.

3. T.T.T. Nguyen, A. Kundan, P.C. Wayner Jr., J.L. Plawsky, D.F. Chao, R.J. Sicker, The effect of an ideal fluid mixture on the evaporator performance of a heat pipe in microgravity, Int. J. Heat Mass Transf. 95 (2016) 765â??772.