High Local Heat Transfer Research Articles

Sidewall Effects on Heat Transfer Coefficient in a Narrow Impingement Channel

This paper examines the local and averaged effects of channel height in the presence of sidewalls on heat transfer coefficients, with special attention given to the sidewall behavior. High resolution local heat transfer coefficient distributions on target and sidewall surfaces were computed using temperature sensitive paint, recorded via a scientific-grade charge-coupled device camera, and compared with available literature. This is important, as many of the major correlations related to impingement channels are directly applicable to wide arrays of jets, for which the influence of the presence of sidewalls is minimal. Streamwise pressure distributions were recorded and used to explain heat transfer trends and to determine the thermal effectiveness of each channel. Results are presented for average jet-based Reynolds numbers between 17,000 and 45,000. All experiments were carried out on a large-scale single-row 15-hole impingement channel (with an X/D of 5, a Y/D of 4, and a Z/D of 1, 3 and 5). It was observed that available correlations accurately predict the target surface heat transfer coefficients when the influence of the sidewall is minimal, typically at larger channel heights and lower Reynolds numbers. Smaller channel heights tend to outperform larger channels, when considering thermal effectiveness and channel-averaged heat transfer rates, due to the increased heat transfer on the channel sidewalls.

Development of modular helium-cooled divertor for DEMO based on the multi-jet impingement (HEMJ) concept: Experimental validation of thermal performance

A modular helium-cooled divertor design based on the multi-jet impingement concept (HEMJ) that is capable of accommodating a surface heat flux of at least 10 MW/m 2 has been developed at the Forschungszentrum Karlsruhe [P. Norajitra, et al., He-cooled divertor for DEMO: experimental verification of the conceptual modular design, Proceedings of the ISFNT-7, May 22–27, 2005, Tokyo, Japan. Fusion Engineering and Design, 81 (1–7) (2006) 341–346]. Thermal-hydraulic simulations used in the original design optimization process predicted extremely high local heat transfer coefficients (∼50 kW/(m 2 K)) [R. Kruessmann, Parametric study to evaluate the design options of the HEMJ divertor cooling concept by CFD simulations (Task TW4-TRP-001). Internal Report, Forschungszentrum Karlsruhe, July 2006]. This prompted an experimental investigation to validate the numerical results. A one-to-one scale test section of the HEMJ divertor reference geometry has undergone testing in an air flow loop under a wide range of operating conditions that include the Reynolds number of the actual helium-cooled HEMJ design. The temperature distribution and local heat transfer coefficients have been measured and compared to numerical predictions of the ANSYS FLUENT ® CFD software package [ANSYS FLUENT ® 6.2.16, ANSYS FLUENT Inc., 2005]. The experimental data closely match the numerical predictions. This comparison provides confidence in using the numerical model for the design of the HEMJ divertor, as well as other gas-cooled high heat flux components. Future experiments will be performed using the same test module with helium cooling at prototypical Reynolds numbers.

Experimental investigation of heat transfer performance for a novel microchannel heat sink

We demonstrated a novel microchannel heat sink with a high local heat transfer efficiency contributed by a complicated microchannel system, which comprises parallel longitudinal microchannels etched in a silicon substrate and transverse microchannels electroplated on a copper heat spreader. The thermal boundary layer develops in transverse microchannels. Meanwhile, the heat transfer area is increased compared with the conventional microchannel heat sink only having parallel longitudinal microchannels. Both benefits yield high local heat transfer efficiency and enhance the overall heat transfer, which is attractive for the cooling of high heat flux electronic devices. Infrared tests show the temperature distribution in the test objects. The effects of flow rate and heat flux levels on heat transfer characteristics are presented. A uniform temperature distribution is obtained through the heating area. The reference temperatures decrease with the increasing flow rate from 0.64 ml min−1 to 6.79 ml min−1 for a constant heat flux of 10.4 W cm−2. A heat flux of 18.9 W cm−2 is attained at a flow rate of 6.79 ml min−1 for assuring the maximum temperature of the microchannel heat sink less than the maximum working temperature of electronic devices.

Highly efficient manifold microchannel heatsink

A manifold microchannel heatsink with high local heat transfer efficiency is presented. CFX simulation indicates that the microchannel heatsink has higher heat transfer efficiency than the conventional structure. Infrared tests demonstrate the dependence of substrate steady-state temperature on flow rate, 0.64–6.7 ml/min, with heat flux of 104000 W/m2. Maximum heat flux input, 203000 W/m2, is obtained for 6.7 ml/min flow rate.

Low Reynolds number forced convection in three-dimensional wavy-plate-fin compact channels: fin density effects

Steady forced convection in periodically developed low Reynolds number (10 ⩽ Re ⩽ 1000) air ( Pr = 0.7) flows in three-dimensional wavy-plate-fin cores is considered. Constant property computational solutions are obtained using finite-volume techniques for a non-orthogonal non-staggered grid. Results highlight the effects of wavy-fin density on the velocity and temperature fields, isothermal Fanning friction factor f, and Colburn factor j. The fin waviness is seen to induce the steady and spatially periodic growth and disruption of symmetric pairs of counter-rotating helical vortices in the wall-trough regions of the flow cross-section. The thermal boundary layers on the fin surface are thereby periodically interrupted, resulting in high local heat transfer near the recirculation zones. Increasing fin density, however, tends to dampen the recirculation and confine it. The extent of swirl increases with flow rate, when multiple pairs of helical vortices are formed. This significantly enhances the overall heat transfer coefficient as well as the pressure drop penalty, when compared to that in a straight channel of the same cross-section. The relative surface area compactness as measured by the ( j/ f) performance or the area goodness factor nevertheless increases with fin density.

Detailed heat transfer characteristic comparison in straight and 90-deg turned trapezoidal ducts with pin-fin arrays

Comparison of endwall heat transfer and pressure drop characteristics is made between the pin-fin trapezoidal ducts with straight and lateral outlet flows. The effect of pin arrangement for the ducts of different-directional outlet flows is also examined. A transient liquid crystal method is used to measure the detailed heat transfer coefficients on the endwall surfaces. It is found that in the straight trapezoidal duct, the local heat transfer coefficient decreases along the spanwise direction from the long sidewall to the short sidewall. In addition, the staggered pin array produces relatively uniform and higher local heat transfer than the in-line pin array. As the flow turns 90° out of the trapezoidal duct, the pin arrangement affects insignificantly the local and endwall-averaged heat transfer. Results further show that the 90° turned trapezoidal ducts inserted with either staggered or in-line pin array perform the best endwall-averaged heat transfer and, meanwhile, pay the largest pressure-drop penalty. The straight trapezoidal duct with in-line pin array, however, has the lowest endwall-averaged heat transfer coefficients as well as the pressure-drop coefficients.

Turbulent Shear Flow and Heat Transfer Over the Repeated Two-Dimensional Square Ribs on Ground Plane

This paper describes the flow structure over the repeated two-dimensional square ribs of side length D, placed at a pitch S on a ground plane. The value of S/D which most augments the turbulence of the free stream and, hence the heat transfer is calculated. The region of interest in this investigation is far downstream where the velocity and temperature distributions follow similarity rules. The time-mean velocity, static pressure, and the velocity vectors were measured by Pivot-and static pressure tubes and a three hole cylindrical yawmeter. The turbulence intensities and integral scale were obtained using a hot wire anemometer. The mean temperature distribution was measured by thermocouples and the local heat transfer coefficient was then calculated. It is found that at S/D=9 the turbulence intensity is maximized. As a result of this effect and the fact that for S/D=9 the flow reattaches within a groove, the heat transfer is also maximized. The measurements show how the location of reattachment depends on S/D and that high local heat transfer coefficient coincides with the reattachment point. The average heat transfer coefficient and the pressure drop correlation is quantified.

Three-Dimensional Natural Convection From Vertical Heated Plates With Adjoining Cool Surfaces

Natural convection in an enclosure with a uniform heat flux on two vertical surfaces and constant temperature at the adjoining walls has been investigated both experimentally and theoretically. The thermal boundary conditions and enclosure geometry render the buoyancy-induced flow and heat transfer inherently three dimensional. The experimental measurements include temperature distributions of the isoflux walls obtained using an infrared thermal imaging technique, while the three-dimensional equations governing conservation of mass, momentum, and energy were solved using a control volume-based finite difference scheme. Measurements and predictions are in good agreement and the model predictions reveal strongly three-dimensional flow in the enclosure, as well as high local heat transfer rates at the edges of the isoflux wall. Predicted average heat transfer rates were correlated over a range of the relevant dimensionless parameters.

Thermosolutal Inducement of No-Slip Free Surfaces in Combined Marangoni-Buoyancy Driven Cavity Flows

The presence of a diffusing surfactant in combined surface tension-buoyancy driven convection is studied numerically to examine its influence on the flow structure and heat transfer characteristics of a fluid contained within a cavity. By accounting for the surface tension dependence on both temperature and surfactant concentration, the free surface may become stagnant. As a result, the high local and overall heat transfer rates at the cold wall, which are associated with thermal surface tension-augmented buoyancy driven flows, are decreased.

The Effects of Turbulence and Stator/Rotor Interactions on Turbine Heat Transfer: Part II—Effects of Reynolds Number and Incidence

Part I of this paper presents airfoil heat transfer data obtained in a rotating turbine model at its design rotor incidence. This portion of the paper presents heat transfer data obtained in the same model for various combinations of Reynolds number and inlet turbulence and for a very wide range of rotor incidence. On the suction surfaces of the first-stage airfoils the locations and lengths of transition were influenced by both the inlet turbulence level and the Reynolds number. In addition it was demonstrated that on the first-stage pressure surfaces combinations of high Reynolds number and high turbulence can produce heat transfer rates well in excess of two-dimensional turbulent flow. Rotor heat transfer distributions indicate that for relatively small deviations from the design incidence, local changes to the heat transfer distributions were produced on both pressure and suction sides near the stagnation region. For extremely large negative incidence the flow was completely separated from the rotor pressure surface, producing very high local heat transfer.

Arrays of Impinging Jets with Spent Fluid Removal through Vent Holes on the Target Surface—Part 1: Average Heat Transfer

Measurements of average convective heat transfer are reported for square arrays of impinging air jets. The target plate on which the jets impinge is perforated so that spent air is withdrawn through the plate rather than at one or more edges of the array, as is usually the case in such investigations. Jet holes and vent holes had the same diameters, but the spacing of the jet holes was twice that of the vent holes. This information is especially relevent to the design of hybrid cooling configurations, in which a surface is cooled by the combined mechanisms of impingement and transpiration. Tests were conducted for both inline arrangements (with a vent hole opposite each jet orifice) and for staggered arrangements; and the latter always yielded higher average heat transfer. The degradation of performance of inline arrays was most pronounced when the clearance between the jet orifice plate and the target plate was small. Under these conditions, a significant portion of each jet flows directly out through the opposing vent without “scrubbing” the target surface. Arrays with staggered vent holes yield heat transfer rates consistently higher (sometimes by as much as 35 percent) than the same jet array with edge venting. The authors attribute the superior performance of the former geometry to high local heat transfer due to boundary layer suction in the vicinities of the vent holes.

Fluid resistance

3D-printed inserts allowing for the tailored guiding of a fluid flow for enhanced heat transfer at low pressure drop are presented. The fluid guiding elements (FGE) introduced in this contribution are based on B-spline surfaces allowing for a dedicated subdivision of the flow in several partial flows which are alternatingly contacted with the heat transferring wall, thereby maintaining a high local heat transfer all along the wall which leads to an enhanced overall heat transfer capacity. By changing the design parameters, the number of partial flows and the individual residence time at the wall can be adjusted to the needs of the considered application. Besides the discussion of the basic design strategy, the concept proposed is validated via simulation and experimental studies. Using the technique of selective laser melting, the produced metallic FGE exhibit walls of only 150 μm thickness thereby imposing only a marginal obstruction of the flow. It is shown that by the use of the FGE the length of a pipe-in-pipe heat exchanger can be drastically reduced.