Heat Transfer In Rectangular Microchannels Research Articles

Investigation of heat transfer in rectangular microchannels

An experimental investigation was conducted to explore the validity of classical correlations based on conventional-sized channels for predicting the thermal behavior in single-phase flow through rectangular microchannels. The microchannels considered ranged in width from 194 μm to 534 μm, with the channel depth being nominally five times the width in each case. Each test piece was made of copper and contained ten microchannels in parallel. The experiments were conducted with deionized water, with the Reynolds number ranging from approximately 300 to 3500. Numerical predictions obtained based on a classical, continuum approach were found to be in good agreement with the experimental data (showing an average deviation of 5%), suggesting that a conventional analysis approach can be employed in predicting heat transfer behavior in microchannels of the dimensions considered in this study. However, the entrance and boundary conditions imposed in the experiment need to be carefully matched in the predictive approaches.

Two-Phase Flow and Heat Transfer in Rectangular Micro-Channels

Knowledge of flow pattern and flow pattern transitions is essential to the development of reliable predictive tools for pressure drop and heat transfer in two-phase micro-channel heat sinks. In the present study, experiments were conducted with adiabatic nitrogen-water two-phase flow in a rectangular micro-channel having a 0.406×2.032mm2 cross-section. Superficial velocities of nitrogen and water ranged from 0.08 to 81.92 m/s and 0.04 to 10.24 m/s, respectively. Flow patterns were first identified using high-speed video imaging, and still photos were then taken for representative patterns. Results reveal the dominant flow patterns are slug and annular, with bubbly flow occurring only occasionally; stratified and churn flow were never observed. A flow pattern map was constructed and compared with previous maps and predictions of flow pattern transition models. Features unique to two-phase micro-channel flow were identified and employed to validate key assumptions of an annular flow boiling model that was previously developed to predict pressure drop and heat transfer in two-phase micro-channel heat sinks. This earlier model was modified based on new findings from the adiabatic two-phase flow study. The modified model shows good agreement with experimental data for water-cooled heat sinks.

Thermal design methodology for high-heat-flux single-phase and two-phase micro-channel heat sinks

This paper explores several issues important to the thermal design of single-phase and two-phase micro-channel heat sinks. The first part of the paper concerns single-phase heat transfer in rectangular micro-channels. Experimental results are compared with predictions based on both numerical as well as fin analysis models. While the best agreement between predictions and experimental results was achieved with numerical simulation, a few of the fin models are found to provide fairly accurate predictions. The second part of the paper focuses on predicting the incipient boiling heat flux. A comprehensive model based on bubble departure and superheat criteria is developed and validated with experimental data. The incipience model is capable of predicting the location, shape and size of bubbles departing in rectangular micro-channels. In the third part of the study, an analytical model is developed to predict pressure drop across a two-phase micro-channel heat sink. This model provides a detailed assessment of pressure drop concerns with two-phase micro-channels, including compressibility, flashing and choking. Overall, the present study provides important guidelines concerning practical implementation of micro-channel heat sinks in high-heat-flux electronic cooling applications.

Heat transfer in rectangular microchannels

Convection heat transfer in a rectangular microchannel is investigated. The flow is assumed to be fully developed both thermally and hydrodynamically. The H2-type boundary condition, constant axial and peripheral heat flux, is applied at the walls of the channel. Since the velocity profile for a rectangular channel is not known under the slip flow conditions, the momentum equation is first solved for velocity. The resulting velocity profile is then substituted into the energy equation. The integral transform technique is applied twice, once for velocity and once for temperature. The results show a similar behavior to previous studies on circular microtubes. The values of the Nusselt number are given for varying aspect ratios.

Slip-flow heat transfer in rectangular microchannels

Laminar slip-flow forced convection in rectangular microchannels is studied analytically by applying a modified generalized integral transform technique to solve the energy equation, assuming hydrodynamically fully developed flow. Results are given in terms of the fluid mixed mean temperature, and both local and fully developed mean Nusselt numbers. Heat transfer is found to increase, decrease, or remain unchanged, compared to non-slip-flow conditions, depending on two dimensionless variables that include effects of rarefaction and the fluid/wall interaction. The transition point at which the switch from heat transfer enhancement to reduction occurs is identified for different aspect ratios.

Modeling forced liquid convection in rectangular microchannels with electrokinetic effects

The effects of the electric double layer near the solid–liquid interface and the flow induced electrokinetic field on the pressure-driven flow and heat transfer through a rectangular microchannel are analyzed in this work. The electric double layer field in the cross-section of rectangular microchannels is determined by solving a non-linear, two-dimensional Poisson–Boltzmann equation. A body force caused by the electric double field and the flow-induced electrokinetic field is considered in the equation of motion. For steady-state, fully-developed laminar flows, both the velocity and the temperature fields in a rectangular microchannel are determined for various conditions. The flow and heat transfer characteristics with⧹without consideration of the electrokinetic effects are evaluated. The results clearly show that, for aqueous solutions of low ionic concentrations and a solid surface of high zeta potential, the liquid flow and heat transfer in rectangular microchannels are significantly influenced by the presence of the electric double layer field and the induced electrokinetic flow.