Optimization of the distance between the vertical plates in the convective air heat exchanger

Abstract

This paper examines the influence of the distance between vertical plates on the intensity of free convective heat transfer along with the optimization of this distance. Experimental tests were carried out for one model channel of such an heat exchanger with widths s=0.045, 0.085 and 0.18 m. This channel, open at the top and sides, was formed by two isothermal symmetrically heated parallel vertical plates of dimensions H=0.5 m and B=0.25 m. The influence of the heating surface temperatures tw=30, 40, 50, 55, 60 and 70 °C on the convective temperature fields and velocity generated inside the channels was investigated. Directly measured temperature fields, as well as velocity fields measured indirectly using the NRP, enabled the thermodynamic parameters of the heat exchanger to be determined. Based on the temperature gradient distribution ∂Tw∂x on the wall, its average value was determined for each of the plates and for the entire channel, after which the heat flux Qw transferred from the plates was calculated. The heat flux transferred with the air Q and the efficiency η=Q/Qw of heat transfer in the channel were determined using the balance method based on the average temperatures and air velocities at the inlet t̄bott and ūin and at the outlet t̄top and ūout of the channel obtained from the temperature and velocity fields. A grid placed vertically in the channel, halfway across the panel width and perpendicular to the heating surfaces was used to detect the temperature field in air. The image and matrix of these temperatures were determined using a thermal imaging camera. The numerical reconstruction procedure (NRP) was used to determine the velocity field.For technical reasons and lack of time it was not possible to test more channels. Since three values of the width s were insufficient to determine sopt, it was decided to use numerical research. For this purpose, numerical results were compared with those obtained experimentally for the channels already tested. As the compliance verification was successful, further studies of the channels were continued numerically. As a result of optimizing the heat exchange intensification, optimal widths of a single channel sopt=0.035 m and n channels s0=0.0123 m, included in the heat exchanger, were obtained.