Numerical Investigation of Turbulence Models for Minichannels

Due to having considerably small diameters compared to the macro channels; validation of conventional models and correlations and, examination of heat transfer and flow characteristics for mini/micro channels have been an attractive subject for last decades. In this study, classical turbulence models are compared and applicability of the conventional correlations is investigated for the flow through minichannels having diameter range between 1.2 and 0.25 mm. For the flow considered, fluid (R134a) enters the horizontal channel with a prescribed temperature and velocity, absorbs heat from the surrounding and then leaves the channel. Reynolds number is chosen in a range between 5000 and 20000 in order to cover the turbulent regime. For the first step of the study, in order to investigate the use of conventional turbulence models, Standard k-ε, RNG k-ε, Realizable k-ε, Standard k-w and Reynolds Stress models are employed to estimate friction factor and Nusselt number values for 0.5 mm diameter channel. These numerical results are compared with those calculated by conventional correlations existing in the literature. According to the comparison, none of the models create a dramatically deviation and Standard k-ε is determined as the model giving the closest results to the conventional values. As second step of the study, Standard k-ε model is applied for the flow through the minichannels having diameter of 1.2, 1, 0.8, 0.5 and 0.25 mm, respectively. Friction factor and Nusselt number values estimated numerically via Standard k-ε model are compared with those calculated by conventional correlations and existing relevant experimental data. According to the study, it is concluded that the numerical friction factor values are found to be close to the conventional values. The most discrepancy exists when diameter is less than and equal to 0.5 mm. Furthermore, numerical Nusselt number values are found to be close to conventional values estimated with the correlation proposed by Gnielinski (1976) while they are lower estimated for channels having diameter of 1.2, 1 and 0.8 mm and over estimated for 0.5 and 0.25 mm diameter channels. As a result, conventional correlations and turbulence models are found to be applicable for the diameter range and the flow investigated.