M. A. Al-Nimr

A THEORETICAL AND EXPERIMENTAL STUDY

A Iheoreticat and experimental model which describes the transient behaviour of a matrix solar air heater is presented. The transient behaviour of the heater results from sudden changes in the intensity of the incident solar radiation and the inlet fluid temperature. The temperature distributions in both the fluid and the solid matrix domains are given exactly, and experiments are conducted to verify the validity of the theoretical model. There was good agreement between the measured and predicted values.

Using Porous Fins for Heat Transfer Enhancement

This work introduces a novel method that enhances the heat transfer from a given surface by using porous fins. The thermal performance of porous fins is estimated and compared with that of the conventional solid fins. It is found that using porous fin of porosity ∈ may enhance the performance of an equal size conventional solid fin and, as a result, save 100 e percent of the fin material. The effect of different design and operating parameters on the porous fin thermal performance is investigated. Examples of these parameters are Ra number, Da number, and thermal conductivity ratio. It is found that more enhancement in the porous fin performance may be achieved as Ra increases especially at large Da numbers. Also, it is found that there is an optimum limit for the thermal conductivity ratio beyond which there is no further improvement in the fin performance.

Improving the performance of double-pipe heat exchangers by using porous substrates

This work introduces a novel method that improves the thermal performance of a conventional concentric tube heat exchanger. The introduced method includes inserting porous substrates at both sides of the inner tube wall. The porous substrates improve the convective heat transfer coefficient between the tube wall and the fluid. This improvement is investigated numerically and its effects on the effectiveness of the heat exchanger are evaluated. The present numerical results show that inserting the porous substrate may enhance the heat exchanger effectiveness considerably for both parallel flow and counter flow arrangements, especially at high values of the heat capacity ratios. In the current study, the effect of inserting porous substrates on the pumping head of the fluid is also investigated.

Enhancing heat transfer in parallel-plate channels by using porous inserts

Abstract In the present work, transient forced convection in the developing region of parallel-plate ducts is numerically investigated. A high-thermal conductivity porous substrate is attached to the inner wall of one plate in order to enhance the heat transfer characteristics of the flow under consideration. The Darcy–Brinkman–Forchheimer model is used to model the flow inside the porous domain. The present study reports the effect of several operating parameters on the flow hydrodynamics and thermal characteristics. Mainly, the current study demonstrates the effects of porous layer thickness, Darcy number, thermal conductivity ratio, and microscopic inertial coefficient on the thermal performance of the present flow. It is found that the highest Nusselt Number is achieved at fully porous duct. Results show that for Darcy number less than 10−4, the effect of microscopic inertial coefficient can be eliminated while for large microscopic inertial coefficient, higher than 103, the effect of Darcy number is observed to be insignificant. Heat transfer can be enhanced by: (1) using high thermal conductivity inserts, (2) decreasing Darcy number, and (3) increasing microscopic inertial coefficient. Also, the study shows that in the developing region, Darcy number and microscopic inertial coefficient have higher effect on the thermal and hydrodynamic behavior of the flow than that in the fully developed region.

Developing Free-Convection Gas Flow in a Vertical Open-Ended Microchannel Filled with Porous Media

Abstract The developing hydrodynamic and thermal behaviors of free convection gas flow in a vertical open-ended parallel-plate microchannel filled with porous media are investigated numerically. The extended Darcy-Brinkman-Forchheimer model is used to model the flow in porous medium and the solid and fluid media are not assumed in local thermal equilibrium. The microflow regime considered is the slip flow regime. The slip in velocity and jump in temperature are found to decrease in the axial direction of the flow. The friction factor is found to decrease as Knudsen number, Forchheimer number and Grashof number are increased. However, the friction factor is found to increase as Darcy number increased. On the other hand, Nusselt number is found to decrease as Knudsen number, Darcy number and thermal conductivity ratio are increased, whereas it increased as Forchheimer number, Grashof number and Biot number are increased.

MAGNETOHYDRODYNAMIC MIXED CONVECTION FROM A VERTICAL PLATE EMBEDDED IN A POROUS MEDIUM

Magnetohydrodynamic mixed convection flow about a vertical flat plate embedded in a porous medium is considered. The effect of the magnetic field strength on the local Nusselt number and local wall shear stress is presented. The non-Darcian model including both the inertial and boundary effects is used. A particular transformation for the governing equations is adopted to cover the whole mixed convection regime within two finite limits. Appreciable effects of the magnetic field strength on the local Nusselt number as well as on the local wall shear stress in the mixed convection regime are found.

On the Analysis of Short-Pulse Laser Heating of Metals Using the Dual Phase Lag Heat Conduction Model

Transient heat conduction in a thin metal film exposed to short-pulse laser heating is studied using the dual phase lag heat conduction model. The initial heat flux distribution in the film, resulting from the temporal distribution function of the laser pulse, together with the zero temperature gradients at the boundaries normally used in literature with the presumption that they are equivalent to negligible boundary heat losses is analyzed in detail in this paper. The analysis presented here shows that using zero temperature gradients at the boundaries within the framework of the dual phase lag heat conduction model does not gcutrantee negligible boundary heat losses unless the initial heat flux distribution is negligibly small. Depending on the value of the initial heat flux distribution, the presumed negligible heat losses from the boundaries can be even way larger than the heat flux at any location within the film during the picosecond laser heating process. Predictions of the reflectivity change of thin gold films due to a laser short heat pulse using the dual phase lag model with constant phase lags are found to deviate considerably from the experimental data. The dual phase lag model is found to overestimate the transient temperature in the thermalization stage of the laser heating process of metal films, although it is still superior to the parabolic and hyperbolic one-step models.

A modified tubeless solar collector partially filled with porous substrate

In this work, the thermal performance of a conventional tubeless collector is improved by inserting porous substrates at the inner side of the collector absorber plate. The porous substrates improve the convective heat transfer coefficient between the absorber plate and the fluid. This improvement is investigated numerically and its effects on the efficiency and the useful gain of the collector are evaluated. It is found that inserting the porous substrate may raise the collector efficiency by 3–32% especially at high values of the overall heat loss coefficient.

Solar Collectors with Tubes Partially Filled with Porous Substrates

In this work, the thermal performance of a conventional collector is improved by inserting porous substrates at the inner walls of the collector tubes. The porous substrates improve the convective heat transfer coefficient between the tube wall and the fluid. This improvement is investigated numerically and its effects on the efficiency and the useful gain of the collector are evaluated. It is found that inserting the porous substrate may raise the collector efficiency considerably, especially at high values of the overall heat loss coefficient.

Transient non-Darcian forced convection flow in a pipe partially filled with a porous material

Abstract In this paper, a numerical simulation is presented for the transient forced convection in the developing region of a cylindrical channel partially filled with a porous substrate. The porous substrate is attached to the inner side of the cylinder wall, which is exposed to a sudden change in temperature. The flow within the porous domain is modeled by the Brinkman-Forchheimer-extended Darcy model. The effects of several parameters on the hydrodynamic and thermal characteristics of the present problem are studied. These parameters include the porous substrate thickness, Darcy number and Forchheimer coefficient. Results of the current model show that the existence of the porous substrate may improve the Nusselt number at the fully developed region by a factor of 8. However, there is an optimum thickness of the porous substrate beyond which no significant improvement in the Nusselt number is achieved. Also, in the present work, the macroscopic inertial term in the porous domain momentum equation is included due to its significant effect. It is found that the steady state time increases as the substrate thickness increases up to a certain limit and then the steady state time decreases upon further increase in the substrate thickness. Also, increasing the Forchheimer coefficient and decreasing the Darcy number increase the steady state time.