Study of fluid flow characteristics at microscale is gaining importance with shrinking device sizes. Better understanding of fluid flow and heat transfer in microchannels will have important implications in electronic chip cooling, heat exchangers, MEMS, and microfluidic devices. Due to short lengths employed in microchannels, entrance header effects can be significant and need to be investigated. In this work, three dimensional model of microchannels, with aspect ratios (a ¼a/b) ranging from 0.1 to 10, are numerically simulated using CFD software tool FLUENT. Heat transfer effects in the entrance region of microchannel are presented by plotting average Nusselt number as a function of nondimensional axial length x*. The numerical simulations with both circumferential and axial uniform heat flux (H2) boundary conditions are validated for existing data set for four wall heat flux case. Large numerical data sets are generated in this work for rectangular cross-sectional microchannels with heating on three walls, two opposing walls, one wall, and two adjacent walls under H2 boundary condition. This information can provide better understanding and insight into the transport processes in the microchannels. Although the results are seen as relevant in microscale applications, they are applicable to any sized channels. Based on the numerical results obtained for the whole range, generalized correlations for Nusselt numbers as a function of channel aspect ratio are presented for all the cases. The predicted correlations for Nusselt numbers can be very useful resource for the design and optimization of microchannel heat sinks and other microfluidic devices. [DOI: 10.1115/1.4004934]

Numerical Investigation of Heat Transfer in Rectangular Microchannels Under H2 Boundary Condition During Developing and Fully Developed Laminar Flow

Prandtl number effect on offset fin heat exchanger performance: predictive model for heat transfer and pressure drop

Numerical investigation on heat transfer behavior of thermally developing flow inside rectangular microchannels

\n The increasingly high power density of today's electronic devices requires the cooling techniques to produce highly effective heat dissipation performance with as little sacrifice as possible to the system compactness. Among the currently available thermal management schemes, convective liquid metal cooling provides considerably high performance due to its unique thermal properties. This paper first reviews the studies on convective cooling using low-melting-point metals published in the past few decades. A group of equations for the thermophysical properties of In-Ga-Sn eutectic alloy is then documented by rigorous literature examination, following by a section of correlations for the heat transfer and flow resistance calculation to partially facilitate the designing work at the current stage. The urgent need to investigate the heat transfer and flow resistance of forced convection of low-melting-point metals in small/mini-channels, typical in compact electronic devices, is carefully argued. Some special aspects pertaining to the practical application of this cooling technique, including the entrance effect, mixed convection, and compact liquid metal heat exchanger design, are also discussed. Finally, future challenges and prospects are outlined.

Convective Cooling of Compact Electronic Devices Via Liquid Metals With Low Melting Points

Forced-convection heat transfer information as a function of the pertinent nondimensional numbers is obtained numerically for laminar incompressible non-Newtonian fluid flow in the entrance region of a square duct with simultaneously developing temperature and velocity profiles for constant axial wall heat flux with uniform peripheral wall temperature. The power-law model characterizes the non-Newtonian behavior. Finite-difference representations are developed for the equations of the mathematical model, and numerical solutions are obtained assuming uniform inlet velocity and temperature distributions. Results are presented for local and mean Nusselt numbers as functions of the Graetz number and the Prandtl number in the entrance region. Comparisons are made with previous analytical work for Newtonian fluids. The results show a strong effect of the Prandtl number on the Nusselt numbers with fully developed and uniform velocity profiles representing the lower and upper limits, respectively. The re...

Laminar Flow and Heat Transfer to a Non-Newtonian Fluid in an Entrance Region of a Square Duct with Prescribed Constant Axial Wall Heat Flux

A numerical method is used to solve the heat transfer equations for laminar flow in ducts of rectangular cross section with simultaneously developing temperature and velocity profiles, both for constant wall temperature and for constant heat input per unit length of the duct. Like the solutions for a fully developed velocity profile, the Nusselt number for each aspect ratio is found to increase from a limiting value at large distances from the entry plane to a maximum at the entry plane. The results also show a strong effect of the Prandtl number on the heat transfer coefficients with uniform and fully developed velocity profiles representing the upper and lower limits respectively. Comparisons are made with analytical solutions for circular ducts and parallel plates and with experimental data.

S. R. Montgomery

Papers

Laminar flow heat transfer for simultaneously developing velocity and temperature profiles in ducts of rectangular cross section