Determination of the Reference Temperature for a Convective Heat Transfer Coefficient in a Heated Tube Bank

The paper presents a theoretical analysis of heat transfer in a heated tube bank, based on the Nusselt number computation as one of the basic dimensionless criteria. To compute the Nusselt number based on the heat transfer coefficient, the reference temperature must be determined. Despite the value significance, the quantity has several different formulations, which leads to discrepancies in results. This paper investigates the heat transfer of the inline and staggered tube banks, made up of 20 rows, at a constant tube diameter and longitudinal and transverse pitch. Both laminar and turbulent flows up to Re = 10,000 are considered, and the effect of gravity is included as well. Several locations for the reference temperature are taken into consideration on the basis of the heretofore published research, and the results in terms of the overall Nusselt number are compared with those obtained by the experimental correlations. This paper provides the most suitable variant for a unique reference temperature, in terms of a constant value for all tube angles, and the Reynolds number ranges of 100–1000 and 1000–10,000 which are in good agreement with the most frequently used correlating equations.

Assessment of turbulence models for low Reynolds number flows and their computational costs, Part 2: Square prism in cross flow

The accuracy of six turbulent flow modeling techniques in an unsteady solution is evaluated against experimental data for a square prism in cross flow. The selected models; SST, SST-SAS, RSM, PANS-SST, DES and LES models are the same as those presented in Part 1 of this study, which focused on flow in a staggered tube bank. For this geometry, the SST model proved to be effective at capturing the averaged Nusselt values per side of the square with relatively low computational costs. The SST model however showed poorer fidelity to the local Nusselt number profile compared to the experimental data. The LES approach provided a more accurate representation of the local Nusselt number but the computational cost was significantly higher. The PANS modification to the SST model did provide a noticeable improvement in accuracy at a reasonable cost while the SAS modification did not see the same improvement. These conclusions are generally consistent with those found for the staggered tube bank in Part 1 of this study. This study can be used as a guide for the industrial user to select a turbulence model for a similar problem with a low Reynolds number and significant flow separation.

Assessment of turbulence models for low Reynolds number flows and their computational costs, Part 1: Staggered tube bank

This study investigates the accuracy of Computational Fluid Dynamics (CFD) models to predict heat transfer in turbulent separated flows at low Reynolds numbers. This article will focus on flow in a staggered tube bank while its companion articular will focus on a square prism (cylinder) in cross flow. Experimental data for both local heat transfer and velocity profiles are available for these cases and have been used extensively in the literature to evaluate various CFD methods. Six unsteady models were used and the results show that the unsteady SST model provided good overall accuracy relative to the mean Nusselt number for both cases. However, the SST model failed to accurately predict local variations. The Partially Averaged Navier-Stokes variant of the SST model did show a marked improvement over the baseline SST model. The Dynamic Smagorinsky Large Eddy Simulation (LES) showed a much-improved fidelity to the local Nusselt number but unpredicted the actual values. The computational cost for the LES model was significant and it was found that the computationally expensive models with higher degrees of resolved turbulence did not necessarily return better results. Finally, the pressure drop results for the six models were scaled to predict the mean Nusselt number with the Generalized Leveque method and was found to be very accurate. This method should prove useful to predict heat transfer performance with computationally less expensive cold flow results.

Experimental Study of a Set of Integrated Cross-Media Heat Exchangers (iCMHXs) for Liquid Cooling in Desktop Computers

The present study is an experimental investigation of a set of five additively-manufactured compact, lightweight, low-cost, air-to-water cross-media heat exchangers suitable for liquid cooling applications in desktop computers, among other applications. The heat transfer between the two fluids is facilitated by solid metallic wires arranged in a staggered tube-bank configuration, in direct contact with both fluids separated by polymer walls. Since the liquid flows externally over the wires instead of flowing inside the tubes in conventional tube-bank fin heat exchangers, smaller wires can be used in iCMHXs, resulting in lighter and more efficient units. The additively-manufactured iCMHX units are post-processed using a conformal polyurethane sealant. The units are experimentally studied in two case studies based on their post-processing techniques. The experimental studies include instrumentation calibration as well as uncertainty analysis. The first case study considers three geometrically identical iCMHX units sharing the same post-processing method. The overall iCMHX performances characterized by the thermal and hydrodynamic parameters, such as thermal resistance and pressure drop for both waterside and airside, are compared. Their experimental results are also compared to 2D CFD predictions. To provide probable reasoning behind the differences in the comparisons, a second case study is then carried out by experimentally investigating two iCMHX units but with variable post-processing approaches such as by using a thinned sealant and by using a single layer of sealant.