Frontiers in Heat and Mass Transfer (FHMT), 8, 23 (2017) DOI: 10.5098/hmt.8.23 Global Digital Central ISSN: 2151-8629 1 EFFECT OF ASPECT RATIO ON SUPERCRITICAL HEAT TRANSFER OF CRYOGENIC METHANE IN ROCKET ENGINE COOLING CHANNELS M. Arun, J. Akhil, K. Noufal, Robin Baby, Darshitha Babu, M. Jose Prakash * Dept. of Mechanical Engineering, TKM College of Engineering, Kollam- 691005, Kerala, India ABSTRACT The supercritical turbulent flow of cryogenic methane flowing in a rocket engine cooing channel is numerically analysed by imposing constant heat flux at the bottom surface of the channel. The calculation scheme is validated by comparing the results obtained with experimental results reported in literature. The heat transfer coefficient is influenced by the strong variation in thermophysical properties of methane at super critical pressure. An increasing trend in the average value of Nusselt number is observed with aspect ratio. The efficacy of both Modified Jackson and Hall and Bishop empirical correlations in predicting Nusselt number is tested for cryogenic methane flowing in coolant channels. Keywords: cryogenic rocket engine, Regenerative cooling, Heat transfer deterioration 1. INTRODUCTION Regenerative cooling is the most commonly used cooling technique used for Liquid Rocket Engine(LRE) thrust chambers. Majority of the rocket engines use fuel as the coolant, which is circulated through the narrow rectangular cooling channels to cool the thrust chamber wall and simultaneously recover a major amount of lost energy of combustion products. The fuel which flows through the channel reaches the injector face, from where it is injected to the combustion chamber at a pressure higher than the chamber pressure. In high pressure rocket engines a small quantity of fuel is injected through small orifices as film coolant to protect the combustion chamber wall from localised heating. In most of the engines the pressures of the fuel in the cooling channels are well above its critical pressure. As the temperature of the fuel flowing at supercritical pressure increases during the regenerative cooling process, it would not undergo a phase change; instead its state changes from a liquid like to a gas like state with drastic variation in thermophysical properties. The temperature at which this phenomenon occurs is known as pseudocritical temperature corresponding to a supercritical pressure. Nowadays methane is considered as an alternative fuel for rocket engine applications. Methane is available in many planets and it can be harvested for carrying out interplanetary missions. The cryogenic methane has like higher density (means smaller storage tank) and higher vaporization temperature compared to cryogenic hydrogen. Methane has higher specific impulse, higher coking limit and superior cooling capability compared to kerosene. The earlier studies on supercritical heat transfer phenomena were mainly focused on fluids like water Yamagata et al. (1974), Koshizuka et al. (1995) Lee et al. (1996) and carbon dioxide Liao et al.(2002), Jiang et al. (2004) He et al.(2005) because of their industrial applications in the field of steam boilers, nuclear reactors and in refrigeration and air conditioning industries. Carlile and Quentmeyr (1992) conducted experimental studies to see the effect of aspect ratio on hot gas side wall temperature. They presented the results for a straight channel with gaseous hydrogen as the coolant and showed that the high aspect ratio cooling passages are showing lesser fatigue damage to the hot gas side wall. Woschnak et al. (2003) developed a new solution strategy, called quasi 2-D approach * Corresponding author. Email: josetkm@tkmce.ac.in which takes into account thermal stratification. Pizzarelli et al. (2007, 2008, 2009a, 2009b, 2010, 2011, 2012, 2013a, 2013b, 2014, 2015) conducted a number of numerical studies to predict coolant flow and heat transfer deterioration in coolant channels with cryogenic methane as the fluid. They developed a simplified model to evaluate the thermal stratification in solid fin as well as the coolant. They also studied the effect of aspect ratio on rectangular as wells as the curved cooling channels to predict the wall temperature and wall heat flux distribution with cryogenic methane as the fluid under transcritical conditions. Through a 2-D axisymmetric study on a circular channel they focussed on heat transfer deterioration aspects of the coolant at transcritical conditions. Urbano et al. (2009) suggested that fluids at transcritical state undergo large changes in properties in the channel. They identified that in addition to the semi empirical correlations available in literature, some other parametric studies will be necessary so as to understand the sensitivity of wall temperature peak. Numerical studies on convective heat transfer of n-heptane was done by Hua et.al (2010) Their results clearly indicate that heat transfer deterioration could occur once the wall temperature reaches pseudocritical temperature. They found that expressions for estimating the heat transfer coefficient for carbon dioxide and water can generally be applied for supercritical of n-heptane when the inlet velocity more than 10 m/s. It is evident from past studies that it is difficult to obtain a unique heat transfer correlation for fluids flowing at supercritical pressures. A modified heat transfer expression which is applicable to the supercritical heat transfer of cryogenic methane flowing inside a horizontal mini tube with constant wall heat flux was established by Wang et.al (2010). The density of cryogenic methane was calculated precisely with the help of a Modified Benedict-Webb-Rubin (MBWR) equation with 15 coefficients. The coefficients of MBWR and thermal conductivity were taken from the models established by Ely and Hanley (1981, 1983). The properties of methane at different supercritical pressures have been reported by Younglove and Ely (1987). Urbano and Nasuti (2012, 2013) analysed the heat transfer capabilities of methane by a parabolized Navier-Stokes solver which includes an accurate equation of state as well as property models, able to describe methane in all thermodynamic states of interest. Ruan and Meng (2012) conducted numerical analyses on the flow and heat transfer of cryogenic methane flowing in a rectangular coolant channel. Their studies do not consider Frontiers in Heat and Mass Transfer Available at www.ThermalFluidsCentral.org