Jaco Dirker

Convective Heat Transfer Coefficients in Concentric Annuli

Abstract The geometric shape of a passages cross-section has an effect on its convective heat transfer capabilities. For concentric annuli, the diameter ratio of the annular space plays an important role. The purpose of this study was to determine to what extent research has been done on convective heat transfer in smooth concentric annuli and, if possible, to improve on or contribute to existing theories. It was found that although various correlations exist, they are not in good agreement. For this study, experiments were conducted with a wide range of annular diameter ratios. The Wilson plot method was used to develop a convective heat transfer correlation for annular diameter ratios of 1.7 to 3.2. For Reynolds numbers (based on the hydraulic diameter), in the range of 4000 to 30000, the deduced correlation predicted Nusselt numbers accurately within 3% of experimental values.

Single-Phase Convective Heat Transfer and Pressure Drop Coefficients in Concentric Annuli

Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers are known to have an effect on their convective heat transfer capabilities. Linear and nonlinear regression models exist for determining the heat transfer coefficients; however, these are complex and time-consuming, and require much experimental data in order to obtain accurate solutions. A large data set of experimental measurements on heat exchangers with annular diameter ratios of 0.483, 0.579, 0.593, and 0.712 with respective hydraulic diameters of 17.01 mm, 13.84 mm, 10.88 mm, and 7.71 mm was gathered. Mean Nusselt numbers were determined using the modified Wilson plot method, a nonlinear regression scheme, and the logarithmic mean temperature difference method. These three methods presented disagreements with existing correlations based on local wall temperatures. The local Nusselt numbers were determined using the logarithmic mean temperature difference method. Local wall temperature measurements were made using a novel method that minimized obstructions within the annulus. Friction factors were calculated directly from measured pressure drops across the annuli. Both heated and cooled horizontal annuli in fully turbulent flow with Reynolds numbers based on the hydraulic diameter varying from 10,000 to 45,000 with water as the working medium were investigated.

Cooling of Power Electronics by Embedded Solids

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.

Convective condensation heat transfer of R134a in tubes at different inclination angles

Abstract An experimental study of convective condensation heat transfer of R134a was conducted in an inclined smooth copper tube of inner diameter of 8.38 mm. The mean vapor quality ranged between 0.1 and 0.9, mass flux between 200 and 400 kg/m2s, inclination angle between −90o (vertical downward) and +90o (vertical upward) at a saturation temperature of 50oC. The results show that the inclination angle and mean vapor quality strongly influence the coefficient of heat transfer. The developed correlation gave an average and mean deviations of 3.44% and 9.22%, respectively, for horizontal flow and, 5.25% and 19.41%, respectively, for vertical downward flow.

Heat Removal from Power Electronics in Two Direction Sets Using Embedded Solid State Cooling Layers―a Proposed Non-Numerical Calculation Method

The use of embedded cooling layers consisting of materials with high thermal conductivities can significantly reduce peak temperatures within solid-state heat-generating media. Inversely, such layers can also allow for increases in heat-generating densities for a given maximum peak temperature. This is applicable in, for instance, integrated passive power electronics, where power densities are limited by the low thermal conductivities of materials being used. In this paper, the thermal performance of embedded cooling layers in three-dimensional rectangular heat-generating components is investigated numerically for a boundary condition where heat escapes to the ambient in two orthogonal direction sets (sets of orthogonal positive and/or negative directions). The allowable increase in heat generation density for fixed maximum peak temperatures is described for a wide range of geometric shape conditions and thermal conductivities of materials present in such composite structures. Correlations were developed for conditions with and without significant thermal resistance at the internal interfaces of the material layers and externally between the composite component structure and the environment. Conventional one-dimensional and first-order approximations traditionally used in composite solid conduction problems can accurately account for neither the relative thickness of material layers, nor the impact that internal interfacial resistance has. This paper presents a method with which the peak temperature within a stacked sandwich structure containing embedded cooling layered and where heat is removed in two orthogonal direction sets can be determined without the use of a numerical package. The method was developed for a wide range of material properties, geometric sizes and interfacial resistance values.

Implementation of Liquid Crystal Thermography to Determine Wall Temperatures and Heat Transfer Coefficients in a Tube-in-tube Heat Exchanger

Liquid crystal thermography was used in a water-operated concentric tube-in-tube heat exchanger to determine local annular heat transfer coefficients at the inlet region. An annular diameter ratio of 0.54 was considered with the inlet and outlet orientated perpendicularly to the axial flow direction. Both heated and cooled cases were considered at annular Reynolds numbers ranging from 1,000 to 13,800. Wall temperature distributions were directly measured by means of a coating of thermo-chromic liquid crystals. Local heat transfer coefficients at the inlet were higher than those predicted by most correlations, but good agreement was obtained with some literature.

Condensing Heat Transfer Coefficients for R134a at Different Saturation Temperatures in Inclined Tubes

This paper presents the effects of saturation temperature and inclination angle on convective heat transfer during condensation of R134a in an inclined smooth copper tube of inner diameter of 8.38 mm. Experiments were conducted for inclination angles ranging from −90° (vertical downward) to +90° (vertical upward) for mass fluxes between 100 kg/m2s and 400 kg/m2s and vapour qualities between 0.1 and 0.9 for saturation temperatures ranging between 30 °C and 50 °C. The results show that saturation temperature and inclination angles strongly influence the heat transfer coefficient. With respect to saturation temperature, an increase in saturation temperature generally leads to a decrease in heat transfer coefficient irrespective of the inclination angle. The effect of inclination angle was found to be more pronounced at mass fluxes of 100 kg/m2s and 200 kg/m2s for the range of vapour qualities considered. Within the region of influence of inclination there is an optimum angle which is between 15° and −30° (downward flow). The inclination effect corresponds to the predominance of the effect of gravity on the flow distribution.© 2013 ASME

Asymmetrical non-uniform heat flux distributions for laminar flow heat transfer with mixed convection in a horizontal circular tube

ABSTRACT Non-symmetric heat flux distributions in terms of gravity in solar collector tubes influence buoyancy-driven secondary flow which has an impact on the associated heat transfer and pressure drop performance. In this study the influence of the asymmetry angle (0°, 20°, 30° and 40°) with regard to gravity for non-uniform heat flux boundaries in a horizontal circular tube was investigated numerically. A stainless steel tube with an inner diameter of 62.68 mm, a wall thickness of 5.16 mm, and a length of 10 m was considered for water inlet temperatures ranging from 290 K to 360 K and inlet Reynolds numbers ranging from 130 to 2000. Conjugate heat transfer was modelled for different sinusoidal type outer surface heat flux distributions with a base-level incident heat flux intensity of 7.1 kW/m2. It was found that average internal heat transfer coefficients increased with the circumferential span of the heat flux distribution. Average internal and axial local heat transfer coefficients and overall friction factors were at their highest for symmetrical heat flux cases (gravity at 0º) and lower for asymmetric cases. The internal heat transfer coefficients also increased with the inlet fluid temperature and decreased with an increase in the external heat loss transfer coefficient. Friction factors decreased with an increase in fluid inlet temperature or an increase in the external heat loss transfer coefficients of the tube model.