Transient Gas to Gas Experimental Study in a Bitubular Configuration Heat Exchanger

This paper presents a comprehensive experimental study of gas-gas flows inside a bitubular (shell-and-tube) heat exchanger (HEX) subjected to various transient conditions, namely (1) increasing or decreasing the internal hot fluid temperature, (2) increasing or decreasing the mass flow rate of the external cold fluid by step variation. Surprisingly, there are very few results in the literature for this last case. However, it seems to be of fundamental interest for some applications. Two types of quantity, the temperatures of the fluids and the heat fluxes exchanged in the HEX, are subject to transient conditions, with a range of time constants from 20 to 80 s. A comparison of experimental results with the main existing analytical model proposed in the literature, essentially validated with liquid-liquid flows, is performed. Deviations from analytical models are given as uncertainties of the time constants. It is shown that one temperature time constant is not suitable for the gas-gas flows. Extension of the results to the heat fluxes responses of the HEX is also performed and confirms the observed results. A sensitivity analysis of the time constants to the main system parameters is reported. It allows the identification of the most important influences and the proposal of primary explanations for the observed results. Local experiments on the same HEX configurations are under development, as well as associated analytical and numerical models. However, the aim is to preserve the simplicity and robustness of the models, to successfully apply them to (real time) control and command of any type of HEX using the equivalent bitubular model. This practical implication, together with the main conclusions of the sensitivity analysis of time constants provides useful insights for modelling and control applications.