Showing papers by "Josua P. Meyer published in 2006"

Heat transfer performance during condensation inside horizontal smooth, micro-fin and herringbone tubes

Abstract: An experimental investigation was conducted into the heat transfer characteristics during in-tube condensation of horizontal smooth, micro-fin, and herringbone tubes. The study focused on the heat transfer coefficients of refrigerants R-22, R-134a, and R-407C inside a series of typical horizontal smooth, micro-fin, and herringbone tubes at a representative average saturation temperature of 40°C. Mass fluxes ranged from 300 to 800 kg/m 2 s, and vapor qualities ranged from 0.85 to 0.95 at condenser inlet, to 0.05 to 0.15 at condenser outlet. The herringbone tube results were compared with the smooth and micro-fin tube results. The average increase in the heat transfer coefficient of the herringbone tube, when compared with the smooth tube at comparable conditions, was found to be 322%, with maximum values reaching 336%. When compared with the micro-fin tube, the average increase in heat transfer coefficient was found to be 196%, with maximum values reaching 215%. Moreover, a new correlation was developed to predict the heat transfer coefficients in a herringbone and micro-fin tube. Semi-local heat transfer coefficients were calculated from the modified Wilson plot technique, using measurements of condenser subsection inlets and outlets, and from knowledge of the temperature gradient on the annulus side. The correlation predicted the semi-local heat transfer coefficients accurately, with 96% and 89% of the data points falling in the ±20% region for the herringbone tube and the micro-fin tube, respectively. The average heat transfer coefficients were accurately predicted, too, with all the data points for the herringbone tube and 83% of the data points for the micro-fin tube falling in the ±20% region. The derived heat transfer correlations can be used for design, especially for reversible heat pumps. This research proves that predicting the flow pattern during intermittent and annular flow is not a prerequisite for predicting the heat transfer accurately to within 20% of the measurements.

Cooling of Power Electronics by Embedded Solids

Abstract: Thermal issues have become a major consideration in the design and development of electronic components. In power electronics, thermal limitations have been identified as a barrier to future developments such as three-dimensional integration. This paper proposes internal embedded cooling of high-density integrated power electronic modules that consist of materials with low thermal conductivity and evaluates it in terms of dimensional, material property, and thermal interfacial resistance ranges. Enhanced component conductivity was identified as a possible economically viable internal cooling option. Thermal performance calculations were performed numerically for conductive cooling of internal component/module regions via parallel-running embedded solids. Thermal advantage per volume usage by the embedded solids was furthermore optimized in terms of a wide range of geometric, material, and thermal parameters. In the dimensional and material property range commonly found in passive power electronic modules, parallel-running cooling layers were identified as an efficient cooling configuration. Numerically based thermal performance models were subsequently developed for parallel-running cooling inserts. A multifunctional experimental setup was constructed to study the cooling of ferrite (operated as a magnetic core) by means of embedded aluminium nitride layers and to verify the thermal model. Results corresponded well with theoretically anticipated performance increases. However, interfacial thermal resistance constituted a major limitation to the cooling performance and future power density increases. With the thermal model developed, functional optimization in terms of magnetic flux density for parallel-running cooling layer configurations was performed for a wide range of material and geometric conditions.

Refrigerant flow regime detection with a capacitance void fraction sensor

Abstract: Flow regime prediction in air-conditioning units is of great importance for designing the inlet distributor and the pass layout of the evaporators/condensers. An important parameter of the two-phase flow inside the tubes is the void fraction. Void fractions and flow regimes are often related in flow pattern maps. In order to refine and expand these maps, more experiments have to be performed. At the Ghent University’s Applied Thermodynamics and Heat Transfer research group a precise void fraction sensor was developed based on the dielectric constant (capacitance) dierence between the gas and fluid phase. Measures were taken to improve the accuracy and reliability of the measurements. An electronic component was developed with a fast response that measures picofarad dierences. The sensor is, due to its inherently simple operating principle, both robust and accurate. The sensor was first tested for air-water flow. An analyses of all dierent flow regimes of the Baker map was made. At the University of Pretoria’s Thermoflow Research Group the sensor was mounted in a testrig for refrigerant two-phase flow (R22) to further analyze the dierent flow regimes. The void fraction signals analysis is used as a way to characterize the flow regimes. In addition, flow patterns were verified using high-speed digital videography.

Refrigerant condensation flow regimes in enhanced tubes and their effect on heat transfer coefficients and pressure drops

Abstract: Flow regimes influence the heat and mass transfer processes during two-phase flow, implying that any statistically accurate and reliable prediction of heat transfer and pressure drop during flow condensation should be based on the analysis of the prevailing flow pattern. Many correlations for heat transfer coefficient and pressure drop during flow condensation completely ignored flow regime effects and treated flows as either annular or non-stratified flow or as stratified flow. This resulted in correlations of poor accuracy and limited validity and reliability. Current heat transfer coefficient, pressure drop, and void fraction models are based on the local flow pattern, though, resulting in deviations of around 20% from experimental data. There are, however, several inconsistencies and anomalies regarding these models, which are discussed in this paper. A generalized solution methodology for two-phase flow problems still remains an elusive goal, mainly because gas-liquid flow systems combine the complex...

Selected Papers from the Third HEFAT Conference

Abstract: The Third International Conference on Heat Transfer, Fluid Mechanics, and Thermodynamics (HEFAT 2004) was hosted in Cape Town from June 21–24, 2004. The conference was a follow-up to HEFAT 2002, wh...

A flow regime map for refrigerant condensation in herringbone micro-fin tubes

Abstract: Experimental and analytical work was performed and visual observations made to investigate the flow regimes during condensation of the refrigerants R-22, R-407C and R-134a in horizontal smooth, helical micro-fin, and herringbone micro-fin tubes at an average saturation temperature of 40°C, with mass fluxes ranging from 300 to 800 kg m -2 s -1 . The flow regimes observed were mainly annular flow and intermittent flow. At low mass fluxes and low average vapour qualities, however, stratified wavy flow was observed. The experimental results show that the transition from annular to intermittent flow regimes occurred at average mass fraction values of 26%, 29% and 48% for the herringbone micro-fin, helical micro-fin, and smooth tubes, respectively. The Thome flow regime transition criterion for annular to intermittent flow, together with the Froude rate and Martinelli parameter, were used to characterize the prevailing flow regime. The combined analyses made it possible to adapt the condensation flow pattern map for helical micro-fin tubes in such a way that it can generate a valid and accurate flow regime map for condensation in herringbone micro-fin tubes. The new transition criterion effectively predicts the delay in the transition from annular to intermittent flow for all three refrigerants condensing in the herringbone micro-fin tube. This new flow regime map has never before been produced for enhanced tubes, and represents an important advance in the field of practical heat-exchanger design.

Failure criteria for polyethylene acetabular cups

Abstract: Information acquired from failed polyethylene acetabular cups used in hip replacements, retrieved from patients, is invaluable to the design engineer in trying to understand how to achieve better in vivo service for these devices. The different failure criteria used by surgeons are vague, as they are primarily intended simply to categorize the failure of an implant. This study proposes a more precise classification based on an evaluation of the materialsbased reasons for failure. The criteria drawn up refer to mechanical damage to the implant, cracks in the material, plastic flow, scratches, adhesion wear and wear particles embedded in the base material, and flaking. An analysis of 47 failed acetabular cups showed that most failures were due to plastic flow of the device material and adhesion wear. These two defects accounted for 62% and 49%, respectively, of the different types of faults observed. Both kinds of failure were caused by localized overheating of the ultra- high-molecular-weight polyethylene used in manufacture. Different types of defects can arise in the same acetabular cup.