Flow Distribution in Brazed Plate Heat Exchangers : A Parameter Study in COMSOL

University essay from Karlstads universitet/Institutionen för ingenjörs- och kemivetenskaper

Abstract:

Lubricants and liquid cooling are used in many industrial applications to ensure reliability and longevity of machinery. Oil cooling of both electrical and mechanical applications is of interest since oil is better suited for electrical applications than water and already available in the system as a lubricant. Brazed plate heat exchangers (BPHEs) have many advantages compared to other heat exchanger types commonly used in oil cooling applications. Flow maldistribution inside BPHEs can arise with highly viscous fluids like oil. Since flow is hard to measure when plate heat exchangers are brazed together, Computational Fluid Dynamics (CFD) can be used instead.

This study investigates parameters that could affect flow distribution inside BPHEs with the CFD-tool COMSOL Multiphysics. The study is made on three different geometries at different detail levels. The purpose of the study is to expand the knowledge about fluid behavior in BPHEs and how it affects efficiency. It was proved from the Bernoulli equation that flow velocity, gravity and Reynolds number were some parameters that could affect flow distribution inside BPHEs.

Two simplified models were built for evaluation of viscosity, gravity and Reynolds number. A more detailed model was provided by SWEP representing the fluid domain of a full-size distribution zone model. Model validation and mesh independence study were made with expressions due to the lack of experimental data. Investigations of viscosity, gravity and Reynolds number were made through isolation and alteration of the respective parameter. The validation and mesh independence study proved the models trustworthy and detailed enough to capture the physical behavior. Small deviations from expected validation results can be explained with the assumptions and simplifications made in the process.

Results show that flow maldistribution increases with viscosity differences between channels. Viscosity maldistribution is greater for oil than for water. It is important to consider how the fluid viscosity changes with temperature under the respective working conditions. Gravity has no effect on flow distribution as long as it acts along or opposite the main flow direction. As plate heat exchangers are generally placed vertically, gravity will not affect flow distribution. Gravity has a significant effect on flow distribution if plate packages are places horizontally. High Reynolds numbers have a positive effect on flow distribution and reduce the difference between highest and lowest velocities across the outlet. Very low flow velocities should therefore be avoided since it increases flow maldistribution.

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