S. Ramadhyani

Conjugate heat transfer in a channel with staggered ribs

Abstract Fluid flow and heat transfer characteristics were analyzed for a constant property fluid flowing laminarly through a parallel plate channel with staggered, transverse ribs and a constant heat flux along both walls. After a finite entry length the flow becomes periodically fully developed. Computations were carried out in the fully-developed regime for different Reynolds numbers, Prandtl numbers, and geometric arrangements. It is found that significant heat transfer augmentation is obtained for high Prandtl number fluids such as water or fluorocarbons. In addition, conduction in the channel walls is found to play a highly beneficial role in enhancing heat transfer.

Convection heat transfer from discrete heat sources in a rectangular channel

Abstract Experiments have been performed to determine convection heat transfer from a single heat source and an in-line, four-row array of 12 heat sources which are flush mounted to one wall of a horizontal, rectangular channel. The experiments were performed with water and FC-77 for channel Reynolds numbers ranging from approximately 1000 to 14,000. Results for a single heat source are in good agreement with those obtained for the first row of the array but exceed predictions based on conventional forced-convection correlations. The average convection coefficient for the rows of the array decreases by approximately 25% from the first to the second row and by less than 5% from the third to the fourth row. The data are in good agreement with model predictions for turbulent flow but are underpredicted for laminar flow.

New correlation to predict the heat transfer coefficient during in-tube cooling of turbulent supercritical CO2

The Nusselt number variations of supercritical carbon dioxide during in-tube cooling are presented and discussed. Using data presented in this paper as well as prior publications, a new correlation to predict the heat transfer coefficient of supercritical carbon dioxide during in-tube cooling has been developed. The new correlation is presented in this paper. It is based on mean Nusselt numbers that are calculated using the thermophysical properties at the wall and the bulk temperatures, respectively. It is seen that the majority of the numerical and experimental values are within ±20% of the values predicted by the new correlation.

Heat Transfer from Supercritical Carbon Dioxide in Tube Flow: A Critical Review

Since the discovery that CFCs and HCFCs destroy the ozone layer and cause global warming, the need to regulate their use has been critical. In the ensuing research for new environmentally benign refrigerants, carbon dioxide has been considered due to its excellent thermophysical properties. This paper gives a review of the heat transfer and pressure drop characteristics of supercritical carbon dioxide in tube flow. This information is necessary for designing the gas cooler of a carbon dioxide refrigeration system. A comparison of the different heat transfer correlations applicable to cooling of supercritical carbon dioxide has also been made.

DEVELOPMENT OF LAMINAR MIXED CONVECTION FLOW IN A HORIZONTAL RECTANGULAR DUCT WITH UNIFORM BOTTOM HEATING

Using a vectorized finite-difference marching technique, the steady-state continuity, momentum, and energy equations are solved numerically to evaluate the effects of buoyancy-induced secondary flow on forced flow in a horizontal rectangular duct with uniform bottom heating. Combined entry region conditions are considered, and the secondary flow is found to consist of longitudinal plumes and vortices that first develop at the vertical sidewalls and subsequently propagate to interior spanwise positions. Sequential stages of the secondary flow development are computed in detail and used to interpret the nonmonotonic longitudinal distribution of the spanwise average Nusselt number. The distribution is characterized by oscillations that, under certain conditions, are damped and yield a fully developed Nusselt number that substantially exceeds the value for pure forced convection.

EXPERIMENTS ON CONVECTIVE HEAT TRANSFER IN CORRUGATED CHANNELS

Experiments have been performed to study convective heat transfer in the entrance region of corrugated channels. With water as the working fluid, two channel spacings were examined for a single corrugation angle of 20°. The flow rate was varied over the range I50 ≤ Re ≤ 4000. Flow visualization under low-Reynolds-number flow conditions suggested the presence of longitudinal vortices, while at somewhat higher Reynolds numbers, the existence of spanwise vortices was clearly revealed. For Re > 1500, Nusselt numbers in the corrugated channels exceeded those in the parallel-plate channel by approximately 140% and 240% for the two channel spacings, the corresponding increases in friction factor being 130% and 280%. Performance evaluations under the criteria of equal mass flow rate, equal pumping power, and equal pressure drop per unit length established both the corrugated channels as superior to the parallel-plate channel in intensifying heat transfer.

Modelling of combustion and heat transfer in a packed bed with embedded coolant tubes

Abstract A model has been developed to simulate combustion and heat transfer in a porous matrix combustor-heater. The system consists of a packed bed in which a premixed natural gas-air mixture combusts. Radiative and convective heat transfer occurs from the flame zone to a fluid flowing through a bank of tubes embedded in the packed bed. Radiative heat transfer in the bed is modelled as a diffusion process, and the flow field and temperature distribution in the bed are calculated. The numerically obtained results are compared with available experimental data for a similar system.

Conjugate heat transfer from small isothermal heat sources embedded in a large substrate

Abstract This paper considers the problem of conjugate heat transfer from discrete heat sources mounted on one wall of a channel and exposed to fully-developed laminar flow. Appropriate dimensionless parameters are suggested by the governing energy conservation equations and boundary conditions, and finite-difference solutions are obtained for selected parameter ranges. Two cases are considered, that of a single heat source and that of two adjoining heat sources. Parametric calculations reveal the range of conditions for which conduction heat transfer from the source to the substrate comprises a significant fraction of the total heat transfer from the source.

CORRELATIONS FOR PREDICTING THE AIR-SIDE NUSSELT NUMBERS AND FRICTION FACTORS IN CHILLED-WATER COOLING COILS

An experimental study was conducted to determine Nusselt numbers and friction factors on the air side of wavy-finned, chilled-water cooling coils. General correlations of the dry-surface Nusselt numbers and friction factors were developed from the data obtained from five different cooling coils. A comparison of the Nusselt number correlation to data from the literature revealed that the correlation was generally able to predict the reported Nusselt numbers within 20%, Under wet-surface conditions, the measured Nusselt numbers showed considerable scatter, with some of the results being higher than the corresponding dry-surface values, while others were lower than the dry-surface values. Friction factors were substantially higher under wet-surface conditions. A correlation to predict Ike friction factors for wet surfaces was also developed.

Combined buoyancy ― and thermocapillary-driven convection in open square cavities

The characteristics of convective flow in a square cavity driven by simultaneous buoyancy and thermocapillary effects have been studied numerically. Results for two sets of thermal boundary conditions suggest that surface tension effects can significantly alter buoyancy-induced flow. The resulting alteration in the flow field can enhance or reduce heat transfer rates across the cavity.