Numerical prediction of thermal performances in a concentric triple tube heat exchanger

Abstract

In the last two decades, many research efforts have been carried out in order to scale down electronic and mechanical devices and components. Under this category also fall power generation devices such as Ultra-Micro Gas Turbines (UMGTs) which are one of the most suitable candidates as portable power supply. In many previous researches the feasibility and reliability of this type of systems have been proven, however many issues still need to be solved. Amongst these there are: excessive heat losses from the hot parts of the devices leading to low thermal efficiency, fluid dynamic losses at microscale need further investigation and the necessity of developing a material able to withstand combined thermal and mechanical stresses. A method to increase thermal efficiency and limiting fuel consumption is to recover heat from the combustion products to preheat the cold incoming mixture. Therefore, this study focuses on the interactions between combustion and heat exchanges at small scale. A numerical model of a three concentric tubes combustion chamber was developed and investigated with the aim of evaluating its thermal performances. A 17 species, 73 reactions skeletal mechanism able to describe methane oxidation was utilized for the purpose. A parametric study was carried out varying the mass flow rate and the thermal conductivity of the walls. Also, the effects of utilizing a flame holder in the combustion chamber were investigated. Results showed how the triple tube combustion chamber allows a good level of regeneration and the inclusion of the flame holder produces additional benefit in terms of heat transferred to the cold mixture.