Spatially-averaged Nusselt Numbers Research Articles

Heat transfer in a twin-blow narrow channel with two opposite walls roughened by skewed ribs arranged in staggered manner

This experimental study performs the detailed heat transfer measurements over a skewed-rib roughened surface in a rectangular narrow channel with two equal-area flow entrances located on two adjacent edges of a channel-corner. This flow configuration allows for increasing the coolant-flow fed into the narrow channel without increasing the height of assembly in order to enhance the capacity of cooling duty for electronic chipsets. At a specified total coolant mass flow rate, four test scenarios, namely the single-blow from the side or upper entrance and the twin-blow with the coolant mass flow ratio of 1 : 1 or 2 : 1 between the side and upper blows are performed. A selection of full-field heat transfer distributions over the rib-roughened surface illustrates the manner by which the flow entry condition and the Reynolds number affect the local and spatially averaged heat transfers. With fixed total coolant consumptions, the twin-blow with the coolant mass flow ratio of 2 : 1 between the side and upper blows elevates the spatially averaged heat transfers to the levels of 150–180% of the single-blow references. A regression-type analysis is subsequently performed to develop the correlation of spatially-averaged Nusselt numbers over rib-roughened surface, which permits the effect of Reynolds number on heat transfer to be determined for four test scenarios.

Spatially Resolved Surface Heat Transfer for Parallel Rib Turbulators With 45 Deg Orientations Including Test Surface Conduction Analysis

Spatially resolved Nusselt numbers, spatially-averaged Nusselt numbers, and friction factors are presented for a stationary channel with an aspect ratio of 4 and angled rib turbulators inclined at 45 deg with parallel orientations on two opposite surfaces. Results are given at different Reynolds numbers based on channel height from 9000 to 76,000. The ratio of rib height to hydraulic diameter is 0.078, the rib pitch-to-height ratio is 10, and the blockage provided by the ribs is 25 percent of the channel cross-sectional area. Nusselt numbers are determined with three-dimensional conduction considered within the acrylic test surface. Test surface conduction results in important variations of surface heat flux, which give decreased local Nusselt number ratios near corners, where each rib joins the flat part of the test surface, and along the central part of each rib top surface. However, even with test surface conduction included in the analysis, spatially-resolved local Nusselt numbers are highest on tops of the rib turbulators, with lower magnitudes on flat surfaces between the ribs, where regions of flow separation and shear layer re-attachment have pronounced influences on local surface heat transfer behavior. The augmented local and spatially averaged Nusselt number ratios (rib turbulator Nusselt numbers normalized by values measured in a smooth channel) decrease on the rib tops, and on the flat regions away from the ribs, especially at locations just downstream of the ribs, as Reynolds number increases. With conduction along and within the test surface considered, globally averaged Nusselt number ratios vary from 3.53 to 1.79 as Reynolds number increases from 9000 to 76,000. Corresponding thermal performance parameters also decrease as Reynolds number increases over this range.

Enhanced heat transfer of forced convective fin flow with transverse ribs

This experimental study investigates the heat transfers in three side-opened and bottom-sealed rectangular channels with two opposite walls roughened by 90° staggered ribs, which simulate the enhanced cooling passages in the fin-type heat sinks of electronic chipsets. The various degrees of interactive effects due to the surface ribs, side-profile leakage flows and streamwise weakened coolant flow are functionally related with Reynolds number ( Re) and channel length-to-gap ratio ( L/ B), which unravel the considerable impacts on local and spatially averaged heat transfers over the rib-roughened fin surfaces. A selection of detailed heat transfer measurements over the rib-roughened fin surfaces illustrates the manner by which the isolated and interactive influences of Re and L/ B-ratio affect the local and spatially averaged heat transfers. Relative to the heat transfer results acquired from the smooth-walled test channels, the augmentations of spatially averaged heat transfers generated by the present surface ribs are in the range of 140–200% of the flat fin reference levels. In conformity with the experimentally revealed heat transfer physics, a regression-type analysis is performed to develop the correlation of spatially-averaged Nusselt number over rib-roughened fin surface, which permits the individual and interactive effect of Re and L/ B on heat transfer to be evaluated. A criterion for selecting the optimal length-to-gap ratio of a fin channel, which provides the maximum convective heat flux from the rib-roughened fin surface, is formulated as an engineering tool to assist the design activity for the cooling device of electronic chipsets.

Variable Property and Temperature Ratio Effects on Nusselt Numbers in a Rectangular Channel With 45 Deg Angled Rib Turbulators

Measured local and spatially-averaged Nusselt numbers and friction factors (all time-averaged) are presented which show the effects of temperature ratio and variable properties in a rectangular channel with rib turbulators, and an aspect ratio of 4. The ratio of air inlet stagnation temperature to local surface temperature Toi/Tw varies from 0.66 to 0.95, and Reynolds numbers based on channel height range from 10,000 to 83,700. The square cross-section ribs are placed on two opposite surfaces, and are oriented at angles of +45 deg and −45 deg, respectively, with respect to the bulk flow direction. The ratio of rib height to channel hydraulic diameter is 0.078, the rib pitch-to-height ratio is 10, and the ribs block 25 percent of the channel cross-sectional area. Ratios of globally-averaged rib Nusselt numbers to baseline, constant property Nusselt numbers, Nu̿/Nuo,cp, increase from 2.69 to 3.10 as the temperature ratio Toi/Tw decreases from 0.95 to 0.66 (provided Reynolds number ReH is approximately constant). Friction factor ratios f/fo,cp then decrease as Toi/Tw decreases over this same range of values. In each case, a correlation equation is given which matches the measured global variations. Such global changes are a result of local Nusselt number ratio increases with temperature ratio, which are especially pronounced on the flat surfaces just upstream and just downstream of individual ribs. Thermal performance parameters are also given, which are somewhat lower in the ribbed channel than in channels with dimples and/or protrusions mostly because of higher rib form drag and friction factors.

Detailed heat transfer measurements of impinging jet arrays issued from grooved surfaces

Heat transfer augmentation of impinging jet-array with very small separation distances ( S/ D j <1) is attempted by using the grooved orifice plate through which the nozzles with different diameters are fitted. The combined effects of groove and nozzle-size distribution in an array have demonstrated considerable influences on heat transfers via their impacts on inter-jet reactions. With a specified coolant flow rate; the detailed heat transfer distributions over the impinging surfaces of three tested arrays are compared to reveal the optimal selections of separation distance and array configuration. Heat transfer modifications caused by varying jet Reynolds number ( Re) and separation distance ( S/ D j ) over the ranges of 1000⩽ Re⩽4000 and 0.1⩽ S/ D j ⩽8 are examined for each test array. In conformity with the experimentally revealed heat transfer physics, a regression-type analysis is performed to develop the correlations of spatially-averaged Nusselt numbers, which permit the individual and interactive effect of Re and S/ D j to be evaluated.