Analysis of flow maldistribution in tubular heat exchangers by fluent

Abstract

One of the common assumptions in basic design theory is that fluid be distributed uniformly at the inlet of the exchanger on each side fluid side and through out the core. However, in practice, flow maldistribution is more common and significantly reduces the desired heat exchanger performance. Maldistribution is the nonuniform distribution of mass flow rate on one or both fluid sides in the heat exchanger core. The major feature of gross flow maldistribution is that non uniform flow occurs at the macroscopic level due to poor header design or blockage of some flow passages during manufacturing, including brazing or operation. Maldistribution causes a significant increase in heat exchanger pressure drop and some reduction in heat transfer rate. In this study, the commercial computational fluid dynamic (CFD) package, Fluent was utilized for modeling the tubular single pass heat exchanger with different tube arrangements namely four tube in-line arrangement, two tube in-line arrangement and square pitch tube arrangement. In each arrangement both flow maldistribution and uniform mass flow distribution are considered. In heat transfer terminology there appear two mean temperatures namely crosssectional mean temperature and adiabatic mean temperature. The cross-sectional temperature is the arithmetic mean of all tube side temperatures and the adiabatic mean temperature is weighted mean of tube side temperatures. The purpose of this investigation is to study the cross sectional mean temperature and adiabatic mean temperature profiles in the computational domain, tubular single pass heat exchanger for flow maldistribution or uniform mass flow distribution on tube side and ideal plug flow on shell side. It is investigated that for uniform mass flow distribution on tube side and ideal plug flow on shell side, there is no difference the cross-sectional mean temperature and adiabatic mean temperature. But for maldistribution with out back flow on tube side and ideal plug flow on shell side, the two mean temperatures have same value at the cross-section ξ=0. For maldistribution with back flow on tube side and ideal plug flow on shell side, the temperature jump occurs at the beginning of the calculation domain. viA large computational effort is involved for the memory access of the computers and computing time for the simulation of the complex geometries associated with the dense grids. The available computational fluid dynamics software package FLUENT is applied to determine the related problems. Standard k - ε turbulence model is allowed to predict the three-dimensional flow and the conjugate heat transfer characteristics.