A Review on CFD Analysis of Turbulent Heat Transfer in Solar Air Heater by using Roughness Geometry

In mechanical industries or thermal power sector the most commonly using element is "AIR PRE HEATER", which is a form of Heat Exchanger and by this equipment we can increase the temperature of the air which has been to use in required purpose. By using the solar air heater we can increase the efficiency of organization. And the use of artificial roughness in the form of repeated ribs on a surface is an effective technique to enhance the rate of heat transfer. A numerical investigation on the heat transfer and fluid flow characteristics of fully developed turbulent flow in a rectangular duct having repeated transverse X shaped rib roughness on the absorber plate has been carried out. The commercial finite-volume CFD code ANSYS FLUENT is used to simulate turbulent airflow through artificially roughened solar air heater. The flow Reynolds number of the duct varied in the range of 3800 to 18,000, most suitable for solar air heater. Solar air heater is operating on the principle of forced convective heat transfer between the wall and a working fluid (air). But the efficiency of the air heater is naturally of low value due to the fact that air has inferior thermodynamics properties in terms of heat transfer but it can be increase by the changing the shape of duct inner area in terms of roughness of artificial roughness. The method used to increase the heat transfer coefficient between the working fluid (air) and absorber surface is to create the turbulence inside the solar air heater duct. The turbulence is used to break the viscous sub layer at the absorber surface and change into the turbulent air flow from the laminar. Yadav and Bhagoria have simulated the above geometries using ANSYS FLUENT code and RNG kε turbulence model. The results were presented in terms of Nusselt number versus Re, friction factor versus Re, Nusselt number ratio versus Re, friction factor ratio versus Re, and thermal enhancement factor versus Re. From the literature review cited above, it is revealed that both experimental and numerical analysis works are done using the Roughness geometries fitted on the flow side of absorber plate. Promvonge et al. have adopted a new concept to increase the heat transfer in solar air heater duct. Promvonge et al. used ribs as artificial roughness on the absorber plate and in addition to this delta winglet, as a swirl flow generator in inlet section to create the additional turbulence from inlet section. This concept results in increase of heat transfer rate.