Transient Heat Transfer during Cryogenic Chilldown

Cryogenic fluids have found many practical applications in today’s world, from cooling superconducting magnets to fueling launch vehicles. In many of these applications the cryogenic fluid is initially introduced into piping systems that are in excess of 150 degrees Kelvin higher than the fluid. This leads to voracious evaporation of the fluid and significant pressure fluctuations, which is accompanied by thermal contraction of system components. This process is known as chilldown, and although it was first investigated more than 4 decades ago, very little data are available on the momentum and energy transport during this transient process. Consequently, the development of predictive models for the pressure drop and heat transfer coefficient has been hampered. In order to address this deficiency, an experimental facility has been constructed that enables the flow structure to be observed while temperatures and pressures at various locations are measured. This study focuses on the inverse numerical procedure used to extract the transient heat transfer coefficient information from the data collected; this information is then used to evaluate the performance of various correlations for heat transfer coefficient in the flow boiling regime. The method developed utilizes flow structure information and temperature measurements, in conjunction with numerical computations for the temperature field within the tube wall, to calculate the heat transfer coefficient. This approach allows the transition point between the film boiling regime and the nucleate boiling regime to be determined, and it also elucidates the variation of the heat transfer coefficient along the circumference of a horizontal tube, with the heat transfer on upper portion being significantly smaller than that at the bottom. INTRODUCTION Cryogenic fluids have been used in various applications throughout the years. One application that has historically been of great interest is the use of cryogenic fluids for rocket propulsion. This interest has been sparked by the fact that “cryogenic propellants are more energetic and environmentally friendly than current storable propellant” [1] and the storage systems for these cryogenic propellants “have an advantage in reduced weight compared to super-critical tanks” [1]. As the propellant tanks are filled, the cryogenic fluid is introduced into a transfer line that is in thermal equilibrium with environment, which results in voracious boiling within the line; this phenomenon is referred to as line “chilldown” or line “cooldown”. Chilldown is characterized by large temperature differences, rapid transients and pressure fluctuations. The phenomenon of chilldown is of interest since it directly impacts the design of delivery systems for the propellant. For example the magnitude of the pressure oscillation determines the thickness of the material used for the transfer lines and the heat transfer rate determines the type and thickness of insulation that is used. Hence a proper understanding of the process allows for a more economical design of cryogenic propellant delivery systems. The vast majority of the studies carried out on chilldown have focused on developing models to predict the time taken to complete chilldown, as well as, predict the temperature and pressure during the process [2-6]. These studies utilize correlations for the heat transfer coefficient that were developed for either steady flows or non-cryogenic flows; neither of which applies to the chilldown process and as a result 1 Copyright © 2005 by ASME only fair agreement is obtained when compared to experimental results. The lack of correlations for heat transfer coefficient during chilldown stems from the fact that directly measuring the heat transfer coefficient during cooldown is difficult. This study focuses on an inverse heat conduction technique that enables the transient heat transfer coefficient during chilldown to be determined. An important feature of this technique is that it accounts for the flow structure in calculating the heat transfer coefficient.