Showing papers by "Ralph L. Webb published in 2000"

Heat transfer and friction characteristics of internal helical-rib roughness

Abstract: This paper provides heat transfer and friction data for single-phase flow in seven 15.54-mm inside diameter tubes having internal helical-rib roughness. The range of geometric parameters were number of rib starts (18 to 45), helix angle (25 to 45 deg), and rib height (0.33 to 0.55 mm). These geometries provide data on a new class of internal enhancement that is typical of commercially rough tubes presently used. The tested geometries provide enhancement by flow separation at the ribs, and by a significant surface area increase. The data were taken with water having 5.08≤Pr≤6.29. Two different correlations were employed to predict the Stanton number and friction factor as a function of geometric variables and Reynolds number. The average deviation of the multiple regression heat transfer and correlations were 2.9 percent and 3.8 percent, respectively. Heat transfer and friction correlations based on the heat-momentum transfer analogy for rough surfaces yielded standard deviations of 1.4 percent and 5.4 percent, respectively. The correlations were shown to reasonably predict the heat transfer and friction for commercially used helical-rib roughened tubes.

Long-Term Hydraulic Performance of Dehumidifying Heat-Exchangers With and Without Hydrophilic Coatings

Abstract: An experimental study has been carried out to investigate the long-term wetting characteristics of dehumidifying finned tube heat-exchangers (coils). The coils were subjected to up to 1000 cycles in which they were alternately dipped into distilled water for 15 minutes and then dried with a fan for 45 minutes. Wetting durability was determined for three uncoated coil, two were coated with proprietary coatings, and a coil with a zinc coating sprayed on the downstream face, and with different fin press oils. Wind tunnel tests on fully wet coils under dehumidification were conducted. The advancing and receding contact angles werer measured by periodically removing small fin samples from the coils. One coating maintained the receding contact angle below 15° throughout the cycle tests. The advancing contact angles were much higher than the receding contact angle. The best coating maintained the wet/dry pressure drop ratio below 1.34 throughout the 1000 cycles. The wet/dry pressure drop ratio was correlated as ...

Wetting Coatings for Dehumidifying Heat Exchangers

Abstract: In dehumidifying heat exchangers, hydrophilic coatings applied on the fins reduce the airside pressure drop under the wet condition. Although coatings have been used for at least a decade, the coating characteristics are not well known. Further, the wetting mechanism of hydrophilic coatings in the literature, and the durability of the coatings is not known. In this paper the types of coating (or surface treatment) are categorized, and the wetting mechanism is described. Dry/wet cycling tests on coupon samples were conducted to test the ability of the coatings to maintain a small contact angle. Cycling sample tests were also conducted to determine the effect of fin press oil on contact angle. Prior to initiating the cycling test, contact angles as small as 10° to 15° were measured. The contact angle of most commercial coatings increased with increasing number of dry/wet cycles, with a maximum in the 55° to 65° range after 1000 cycles. The uncoated aluminum surface showed improved wettability with increased...

Identification of minimum air flow design for a desktop computer using CFD modeling

Abstract: This paper reports the results of CFD analysis to cool the 30-W socketed CPU of a desktop computer with minimum air flow rate and minimum heat sink size. This was achieved using only the fan in the power supply for all air movement in the chassis. A duct was employed to direct the air flow over the CPU and then to the inlet air vents of the power supply. Use of this duct allowed more than 10/spl deg/C reduction of the CPU case temperature, relative to a unducted design. The CFD analysis results were confirmed by experiment, and the predicted CPU case temperatures agreed within /spl plusmn/2.9/spl deg/C of the experimental values for the ducted cases. This paper describes the methodology of CFD analysis for the heat sink/duct design, and describes experimental procedures to validate the predictions.

Effect of Oil On the Boiling Performance of Structured and Porous Surfaces

Abstract: The effect of geometric parameters and compressor oil concentration on the performance degradation of enhanced surfaces were investigated. Data are presented for innovative structured and porous surfaces. Refrigerant R-134a at 26.6°C with 1%, 2.5%, and 5% polyester-ester oil concentration by mass was used as the working fluid. Pore diameter was not found to be a major factor for the oil degradation of boiling performance on structured surfaces and that the smaller pore pitch yielded slightly greater oil degradation. Tubes with 50 fins/in (0.51 mm fin spacing) and 0.9 mm fin height had a smaller oil degradation than the Turbo-B surface. The rectangular fin base substantially enhanced the boiling performance in pure refrigerant but in the oil presence, this geometry performed poorly. The effect of oil degradation increased as the tunnel cross section area decreases. For 5% oil concentration, the porous surface had a performance degradation similar to the Turbo-B tube.