A study on fluid flow and heat transfer in rectangular microchannels with various longitudinal vortex generators

The paper presents both three and two-dimensional numerical analysis of convective heat transfer in microchannels. The three-dimensional geometry of the microchannel heat sink followed the details of the experimental facility used during a previous research step. The heat sink consisted of a very high aspect ratio rectangular microchannel. Two channel heights, namely 1mm and 0.3mm (0.1mm), were used for 3D (2D) numerical model respectively. Water was employed as the cooling liquid. The Reynolds number ranged from 200 to 3000. In the paper, the thermal entrance effect is analyzed in terms of heat transfer efficiency. Finally, the comparison between measured and computed heat flux and temperature fields is presented. Contrary to the experimental work, the numerical analysis did not reveal any significant scale effect in heat transfer in microchannel heat sink up to the smallest size considered (0.1 mm).

The paper presents both three and two-dimensional numerical analysis of convective heat transfer in microchannels. The three-dimensional geometry of the microchannel heat sink followed the details of the experimental facility used during a previous research step. The heat sink consisted of a very high aspect ratio rectangular microchannel. Two channel spacings, namely 1 mm and 0.3 mm (0.1 mm), were used for three-dimensional (two-dimensional) numerical model, respectively. Water was employed as the cooling liquid. The Reynolds number ranged from 200 to 3000. In the paper, thermal entrance effects and conduction/convection coupling effects are considered both for the test case of uniform channel inlet conditions and the complete geometry of the experiment. Finally, the comparison between measured and computed heat flux and temperature fields is presented. Contrary to the experimental work, the numerical analysis did not reveal any significant scale effect on heat transfer in microchannel heat sink down to the smallest size considered (0.1 mm).

Heat and fluid flow in a rectangular channel heat sink equipped with longitudinal vortex generators have been numerically investigated in the range of Reynolds numbers between 25 and 200. Aqueous solutions of carboxymethyl cellulose (CMC) with different concentrations (200–2000ppm), which are shear-thinning non-Newtonian liquids, have been utilised as working fluid. Three-dimensional simulations have been performed on a plain channel and a channel with five pairs of vortex generators. The channels have a hydraulic diameter of 8mm and are heated by constant wall temperature. The vortex generators have been mounted at different angles of attack and locations inside the channel. The shear-thinning liquid flow in rectangular channels with longitudinal vortex generators are described and the mechanisms of heat transfer enhancement are discussed. The results demonstrate a heat transfer enhancement of 39–188% using CMC aqueous solutions in rectangular channels with LVGs with respect to a Newtonian liquid flow (i.e. water). Additionally, it is shown that equipping rectangular channels with LVGs results in an enhancement of 24–135% in heat transfer performance vis-à-vis plain channel. However, this heat transfer enhancement is associated with larger pressure losses. For the range of parameters studied in this paper, increasing the CMC concentration, the angle of attack of vortex generators and their lateral distances leads to an increase in heat transfer performance. Additionally, heat transfer performance of rectangular channels with longitudinal vortex generators enhances with increasing the Reynolds number in the laminar flow regime.

An experimental study on fully developed laminar flow and heat transfer in rectangular microchannels

Experimental and numerical analysis were performed to evaluate heat transfer characteristics of water through five sets of rectangular microchannels. Microchannels of hydraulic diameter of 222 μm, 267 μm, 323 μm, 330 μm and 343 μm respectively for Re number between 2.1 to 48 were analysed. Numerical analysis and experiments were conducted under a input heat of 10 W to 100 W, inlet fluid temperature 29°C and mass flow rates of 0.0167 kg/sec to 0.116 kg/sec. Nusselt number tends to be linear increasing as Reynolds number increases. Besides high temperature gradient exist in the region between inlet and exit of flow microchannel. For hydraulic diameter 222 μm, heat transfer coefficient seems to be higher as compare to other configurations of hydraulic diameters. Numerical results showed reasonably good agreement within 4-5% with experimental results.

A 180-deg turning channel can be seen in many heat exchange systems, and a strong three-dimensional flow phenomenon will occur at the turning point. In order to strengthen the heat transfer inside the turn and considering the lack of relevant enhanced endwall heat transfer approach, the present study innovatively demonstrates the measures of enhanced internal endwall heat transfer using arrays of delta-winglet and conical vortex generators. The studied parameters of the vortex generators include aspect ratio (AR), spacing, and angle of attack, etc. The experimental results indicate that the longitudinal vortex generators have more advantages in increasing the heat transfer than the transverse conical vortex generators in certain configurations with the same order of magnitude of the increased area. Besides, transverse conical vortex generators are more helpful to improve the endwall heat transfer uniformity than longitudinal vortex generators. The array of longitudinal vortex generators with 45-deg angle of attack, 0.02 m pitch, and 2 aspect ratio provides the highest thermal performance for all studied cases, indicating that the longitudinal delta-winglet vortex generators can induce near-wall flow turbulence as well as convective heat transfer of the endwall with a modest pressure loss penalty. For this optimally designed endwall, the Nusselt number is augmented by up to 35%. From the perspective of energy saving, the present studied vortex generators provide a superior thermal performance at identical pumping power, which can be useful in the future design of endwalls in sharp-turn channels.

Numerical study on thermoelectric–hydraulic performance of a thermoelectric power generator with a plate-fin heat exchanger with longitudinal vortex generators

Simulation of thermoelectric-hydraulic performance of a thermoelectric power generator with longitudinal vortex generators

Experiments on the Laminar/Turbulent Transition of Liquid Flows in Rectangular Microchannels

A combined experimental and numerical study on the laminar-to-turbulent transition in microchannels using gas flow is presented. The effects of two geometric parameters, namely aspect ratio (height to width) of microchannels and inlet manifold shape, are considered on the value assumed by the critical Reynolds number linked to the laminar-to-turbulent transition. To study the effect of aspect ratio, seven rectangular microchannels having an aspect ratio between 0.25 and 1.04 are micro-milled in PMMA plastic with a constant length of 100 mm. Four rectangular microchannels with different inlet shapes, namely sudden contraction, rounded entrance, V shape and bellmouth, are fabricated to analyze the effects of inlet shape. Pressure loss analyses are then performed for all 11 microchannels by evaluating both average and semi-local friction factors. The Reynolds number in correspondence of which the transition takes place is determined by observing the trend of the friction factor. In parallel, numerical simulations using an intermittency-based transitional turbulence model are also performed and results are compared with the experiments. Experimental and numerical results have demonstrated that both of the investigated geometrical characteristics (aspect ratio and inlet manifold shape) play an important role on the range of the Reynolds number between the onset of transition and the onset of fully turbulent regime for gas microflows. Experimental critical Reynolds numbers show a good agreement with the predictions of the conventional theory and are in the range of 1863–3470 for all the tested microchannels. The role of gas compressibility on the laminar-to-turbulent transition is also discussed.Graphic abstract

Conjugate heat transfer in rectangular, trapezoidal and rectangular-trapezoidal combined microchannels is investigated for hydro-dynamically fully developed, single-phase, laminar flow with no-slip conditions. Hydraulic diameter of all geometries is so adjusted so that for given aspect ratio of the rectangular microchannel the other two geometries have same hydraulic diameter. Various physical quantities heat transfer coefficient, Temperature gradient has been examined to deduce the effective cooling in the microchannel. Simulation has also been done for various velocities to have a look on effectiveness of the microchannel.

Experiments are performed to investigate the laminar-turbulent transition for the flow of dilute polyacrylamide solutions (PAM) through rectangular microchannels of height 300μm and width 3000μm. Pressure drop is measured across the developed region of the microchannel to obtain friction factor and pressure fluctuation data. The departure of the friction factor data from its laminar value is considered the beginning of a transition in the fluid flow. The transition onset occurs at a much lower Re for the flow of dilute PAM solutions compared to its Newtonian counterpart. Here, Re is the Reynolds number defined as Re=Dh×v×ρ/η, where Dh is the hydraulic diameter, v is the cross-sectional average velocity, ρ is the fluid density, and η is the dynamic fluid viscosity. The hydraulic diameter, Dh, is defined as Dh=4(W×H)/(2(W+H)), where W is the channel width and H is the channel height. Pressure fluctuations and dye stream visualization experiments also show that the flow of dilute PAM solutions undergoes a laminar-turbulent transition at a much lower Re compared to a Newtonian fluid. The PAM concentration is fixed at 20 ppm, and the solvent is made of glycerin solutions with a glycerin concentration in water ranging from 1% to 40%. For the highest glycerin concentration of 40%, the transition onset occurs at Rec∼90. The critical Reynolds number, Rec, when plotted with the elasticity number, E(1−β), for different glycerin concentrations, shows a scaling relationship of Rec∼(E(1−β))−2/5.

Investigation on the heat transfer and energy-saving performance of microchannel with cavities and extended surface

Flow characteristics and heat transfer performances in a rectangular microchannel with dimples/protrusions are studied numerically in this research. The height and the width of the microchannel is 200 μm and 50 μm, respectively. The dimple/protrusion diameter is 100 μm, and the depth is 20 μm. The effects of Reynolds number, streamwise pitch, and arrangement pattern are examined. The numerical simulations are conducted using water as the coolant with the Reynolds number ranging from 100 to 900. The results show that dimple/protrusion technique in mcirochannel has the potential to provide heat transfer enhancement with low pressure penalty. The normalized Nusselt number is within the range from 1.12 to 4.77, and the corresponding normalized friction factor is within the range from 0.94 to 2.03. The thermal performance values show that the dimple + protrusion cases perform better than the dimple + smooth cases. The flow characteristics of the dimples/protrusions in microchannel are similar to those in conventional channel. Furthermore, from the viewpoint of energy saving, dimples/protrusions in microchannel behave better than those in conventional channel. Also from the viewpoint of field synergy principle, the synergy of the dimple + protrusion cases are much better than the dimple + smooth cases. Moreover, the synergy becomes worse with the increase in the Reynolds number and decrease in the streamwise pitch.

Analytical modeling of electrokinetic effects on flow and heat transfer in microchannels