Majid Molki - Academia.edu (original) (raw)
Papers by Majid Molki
Bulletin of the American Physical Society, Nov 23, 2014
International Journal of Engineering, Feb 1, 1994
A review of recent work on flow boiling of CO{sub 2} in microchannels indicated that the heat tra... more A review of recent work on flow boiling of CO{sub 2} in microchannels indicated that the heat transfer coefficient for flow boiling in microchannels is substantially higher than that in tubes of normal size. These characteristics might suggest an extended use of CO{sub 2} in the next century. The authors present their preliminary results obtained for flow boiling of CO{sub 2} in microchannels. These preliminary experiments were conducted at mass fluxes of 250 to 700 kg/m{sup 2}{center_dot}s and heat fluxes of 8 to 25 kW/m{sup 2}. For the range of parameters tested, it was demonstrated that the heat transfer coefficient of CO{sub 2} in microchannels is almost independent of heat flux and mass flux. This might imply that the heat transfer mechanism of CO{sub 2} in microchannels depends on other parameters such as the size and the geometry of the microchannels. Furthermore, besides its excellent thermal properties and environmental advantages, CO{sub 2} offers outstanding heat transfer characteristics compared to traditional refrigerants such as R-134a.
IEEE Transactions on Geoscience and Remote Sensing, 1994
An experimental investigation is carried out to study heat transfer in the entrance region of an ... more An experimental investigation is carried out to study heat transfer in the entrance region of an array of rectangular heated blocks. The focus of the work is on the entrance heat transfer coefficients and the associated thermal wake effects. The experiments were performed with air as the working fluid. The adiabatic heat transfer coefficients and thermal wake effects are correlated for the entrance region. These correlations are incorporated into a user-friendly FORTRAN program which can be used by the engineers to predict the working temperatures of the modules of circuit boards with similar layout. A typical compute output indicated that the mean deviation between the emasured and predicted temperatures is 11.0%
Volume 8: Fluids Engineering; Heat Transfer and Thermal Engineering, Oct 30, 2022
A computer central processing unit (CPU) cooler keeps the CPU at a safe operating temperature. Sp... more A computer central processing unit (CPU) cooler keeps the CPU at a safe operating temperature. Specifically, this study investigates an air-cooled CPU cooler. Heat pipes carry the CPU’s heat to an array of plate fins which eventually transfer the thermal energy to the outside air. Convective heat transfer in the inter-fin air gaps and conduction within the fins make this cooling a conjugate heat transfer process. Thus, the selected geometry’s conjugate heat transfer solution provides a Nusselt number correlation. The location of the heat pipe is a subtle and significant factor that affects the heat rate at those boundaries and is rarely available in the literature. Depending on the location of heat pipes for a certain Reynolds number, the heat rate could be as high as 3.75% more than the heat rate due to their original locations. A high heat rate indicates an increase in heat transfer for a heat pipe location relative to another. Ultimately, this investigation predicts an optimal geometric arrangement for the location of the heat pipes. The optimal layout of heat pipes is essential to designing thermally efficient CPU coolers.
Volume 8: Fluids Engineering; Heat Transfer and Thermal Engineering, Oct 30, 2022
This study is a computational investigation of the interference between a reflected shock wave an... more This study is a computational investigation of the interference between a reflected shock wave and the wall boundary layers formed by gas flow in a shock tube. Two gases, namely, Argon and Helium, are examined as the working fluid in each case at different pressure ratios. The present computations consider the viscous effects and implement turbulence through the Spalart-Allmaras model. The study observes and analyzes the non-ideal transient behavior in the shock tube. It also examines thermal effects and the shock bifurcation for two gases at different pressure ratios of 10 and 100. The present simulations explain the temperature distribution behind the bifurcated shock waves. The results show that the range of disturbance formation for each gas varies with pressure ratio. The outcomes of this study agree well with theoretical methods, which assume uniformity behind the reflected shock wave under ideal conditions. However, consistent with the past studies, this investigation confirms that reflected shock waves interact with the wall boundary layers, and bifurcation occurs. We noticed the gas pressure ratio plays a significant contribution to the tendency and strength of the bifurcation. The higher the pressure ratio, the faster the shock wave traveled, and the quicker the relative bifurcated foot velocities increased with the Mach number at that region. This research contributes and sheds light on the role of gas type and other parameters on the nature of the interaction between a traveling shock wave and a thin boundary layer. This investigation’s findings will benefit the supersonic compressible flow applications and experiments.
ABSTRACT Heat transfer in rectangular channels can be significantly enhanced by formation of seco... more ABSTRACT Heat transfer in rectangular channels can be significantly enhanced by formation of secondary flows. Secondary flow fields appear within the channels and influence the boundary layer growth and improve the convective heat transfer. When a high potential is applied to two electrodes, the consequent high electric field in the gap between the electrodes may exceed the partial break-down limit of the gas molecules. The neutral gas molecules are ionized close to the emitting electrode and accelerate in the direction of the electric field. The accelerating ions impose an electric body-force to the gas and induce a bulk flow. Depending on the location and geometry of the electrodes, the electrically-induced flow field might have different specifications. If the electrodes are laid on the opposite walls of channel and extended in the longitudinal direction, the electric body-force would cause a secondary flow on the cross section of the channel. The electrically-induced flow field disturbs the boundary layer and enhances the convective heat transfer coefficient. However, the enhancement level is more remarkable in natural convection. In this study, the influence of a corona jet on heat transfer in rectangular channels with flat and longitudinal electrodes will be studied. The emitting and collecting electrodes are metallic strips attached to opposite walls of the channel and are extended along the axis of the channel. The electric field governing equations are solved numerically using finite-volume method and the third-order QUICK scheme is utilized for discretization of the charge fluxes. The distribution of electric field and charge density on the cross section of the channel is obtained to find the electric body-force at each point. In the presence of electric and buoyancy forces, the momentum and energy equations are solved to determine the level of enhancement of convective heat transfer using corona discharge.
An experimental study was performed to qualitatively investigate the effect of an applied electri... more An experimental study was performed to qualitatively investigate the effect of an applied electric field on two-phase flow patterns in convective flow boiling of R-134a inside a channel. The experiments were performed in a horizontal channel with 3 mm hydraulic diameter. Inlet vapor qualities of 5%, 40%, and 80% at a flow Reynolds number of 1000 and heat flux of 20 kW/m2 are reported here. The flow regimes were analyzed by a high-speed digital camera. The camera could provide a frames speed of up to 3025 frames/s. The electric field was maintained between the upper and lower walls of the channel. The current visualization study suggests some definitive characteristics of the flow regime and the mechanism behind the EHD-enhanced convective boiling in a narrowly-spaced channel. It is found that at the lower range of applied potential, the flow patterns change from stratified wavy flow to a plug flow type with large slugs spanning the height of the channel. At the higher range of applied potential, a new flow regime took place. This flow pattern was associated with the cross motion of the liquid inside the channel. One reason for heat transfer enhancement can be attributed to the strong heat/momentum transfer between the vapor slugs and liquid cross motions.
Microchannels have been widely used by the refrigeration/air conditioning industry. Zhao et al. (... more Microchannels have been widely used by the refrigeration/air conditioning industry. Zhao et al. (2000a,b) presented their experimental results for CO2 flow boiling in microchannels. In the present study, empirical modeling is performed to correlate the experimental data. Five state-of-the-art, large-tube empirical models are compared with the experimental data. It is found that these correlations fail to satisfactorily predict the experimental data for microchannels. This indicates that the heat transfer behavior in microchannels is different from that in large channels. A new correlation is developed based on Liu and Winterton’s correlation by introducing confinement factor to account for the micro-scale effects. The new model agrees well with the experimental results for CO2 flow boiling in microchannels within ±20%.
Heat Transfer Engineering, 2006
Corona discharge in air was used to generate a secondary flow that significantly improved the con... more Corona discharge in air was used to generate a secondary flow that significantly improved the convective heat transfer coefficient in the developing region of a square duct. A wire electrode, charged with a high voltage, was placed at the center of the duct. The electrode ionized the air and created a corona current, which was measured experimentally and used in
International Journal of Engineering, Nov 1, 1988
International Journal of Heat and Mass Transfer, 1986
Experiments were conducted to determine the axial distribution of per-facet and per-cycle heat tr... more Experiments were conducted to determine the axial distribution of per-facet and per-cycle heat transfer coefficients for turbulent flow in an isothermal, corrugated-wall duct with variable interwall spacing. The investigation was conducted via a mass transfer technique. The focus of the ...
Heat Transfer Engineering, Apr 1, 2007
The predication of nonlinear response of steel buildings under elevated temperatures from fire lo... more The predication of nonlinear response of steel buildings under elevated temperatures from fire loading can be achieved by sequentially using heat-transfer analysis and stress analysis. A combined computational fluid dynamics (CFD) and structural stress ...
Science And Technology For The Built Environment, Jul 1, 2002
EHD-enhanced condensation of alternative refrigerants in smooth and corrugated tubes. A Gidwani, ... more EHD-enhanced condensation of alternative refrigerants in smooth and corrugated tubes. A Gidwani, M Molki, MM Ohadi HVAC & R Research 8:33, 219-237, 7/2002. An experimental study of EHD-enhanced in-tube condensation of alternative refrigerants is presented. ...
Bulletin of the American Physical Society, Nov 23, 2014
International Journal of Engineering, Feb 1, 1994
A review of recent work on flow boiling of CO{sub 2} in microchannels indicated that the heat tra... more A review of recent work on flow boiling of CO{sub 2} in microchannels indicated that the heat transfer coefficient for flow boiling in microchannels is substantially higher than that in tubes of normal size. These characteristics might suggest an extended use of CO{sub 2} in the next century. The authors present their preliminary results obtained for flow boiling of CO{sub 2} in microchannels. These preliminary experiments were conducted at mass fluxes of 250 to 700 kg/m{sup 2}{center_dot}s and heat fluxes of 8 to 25 kW/m{sup 2}. For the range of parameters tested, it was demonstrated that the heat transfer coefficient of CO{sub 2} in microchannels is almost independent of heat flux and mass flux. This might imply that the heat transfer mechanism of CO{sub 2} in microchannels depends on other parameters such as the size and the geometry of the microchannels. Furthermore, besides its excellent thermal properties and environmental advantages, CO{sub 2} offers outstanding heat transfer characteristics compared to traditional refrigerants such as R-134a.
IEEE Transactions on Geoscience and Remote Sensing, 1994
An experimental investigation is carried out to study heat transfer in the entrance region of an ... more An experimental investigation is carried out to study heat transfer in the entrance region of an array of rectangular heated blocks. The focus of the work is on the entrance heat transfer coefficients and the associated thermal wake effects. The experiments were performed with air as the working fluid. The adiabatic heat transfer coefficients and thermal wake effects are correlated for the entrance region. These correlations are incorporated into a user-friendly FORTRAN program which can be used by the engineers to predict the working temperatures of the modules of circuit boards with similar layout. A typical compute output indicated that the mean deviation between the emasured and predicted temperatures is 11.0%
Volume 8: Fluids Engineering; Heat Transfer and Thermal Engineering, Oct 30, 2022
A computer central processing unit (CPU) cooler keeps the CPU at a safe operating temperature. Sp... more A computer central processing unit (CPU) cooler keeps the CPU at a safe operating temperature. Specifically, this study investigates an air-cooled CPU cooler. Heat pipes carry the CPU’s heat to an array of plate fins which eventually transfer the thermal energy to the outside air. Convective heat transfer in the inter-fin air gaps and conduction within the fins make this cooling a conjugate heat transfer process. Thus, the selected geometry’s conjugate heat transfer solution provides a Nusselt number correlation. The location of the heat pipe is a subtle and significant factor that affects the heat rate at those boundaries and is rarely available in the literature. Depending on the location of heat pipes for a certain Reynolds number, the heat rate could be as high as 3.75% more than the heat rate due to their original locations. A high heat rate indicates an increase in heat transfer for a heat pipe location relative to another. Ultimately, this investigation predicts an optimal geometric arrangement for the location of the heat pipes. The optimal layout of heat pipes is essential to designing thermally efficient CPU coolers.
Volume 8: Fluids Engineering; Heat Transfer and Thermal Engineering, Oct 30, 2022
This study is a computational investigation of the interference between a reflected shock wave an... more This study is a computational investigation of the interference between a reflected shock wave and the wall boundary layers formed by gas flow in a shock tube. Two gases, namely, Argon and Helium, are examined as the working fluid in each case at different pressure ratios. The present computations consider the viscous effects and implement turbulence through the Spalart-Allmaras model. The study observes and analyzes the non-ideal transient behavior in the shock tube. It also examines thermal effects and the shock bifurcation for two gases at different pressure ratios of 10 and 100. The present simulations explain the temperature distribution behind the bifurcated shock waves. The results show that the range of disturbance formation for each gas varies with pressure ratio. The outcomes of this study agree well with theoretical methods, which assume uniformity behind the reflected shock wave under ideal conditions. However, consistent with the past studies, this investigation confirms that reflected shock waves interact with the wall boundary layers, and bifurcation occurs. We noticed the gas pressure ratio plays a significant contribution to the tendency and strength of the bifurcation. The higher the pressure ratio, the faster the shock wave traveled, and the quicker the relative bifurcated foot velocities increased with the Mach number at that region. This research contributes and sheds light on the role of gas type and other parameters on the nature of the interaction between a traveling shock wave and a thin boundary layer. This investigation’s findings will benefit the supersonic compressible flow applications and experiments.
ABSTRACT Heat transfer in rectangular channels can be significantly enhanced by formation of seco... more ABSTRACT Heat transfer in rectangular channels can be significantly enhanced by formation of secondary flows. Secondary flow fields appear within the channels and influence the boundary layer growth and improve the convective heat transfer. When a high potential is applied to two electrodes, the consequent high electric field in the gap between the electrodes may exceed the partial break-down limit of the gas molecules. The neutral gas molecules are ionized close to the emitting electrode and accelerate in the direction of the electric field. The accelerating ions impose an electric body-force to the gas and induce a bulk flow. Depending on the location and geometry of the electrodes, the electrically-induced flow field might have different specifications. If the electrodes are laid on the opposite walls of channel and extended in the longitudinal direction, the electric body-force would cause a secondary flow on the cross section of the channel. The electrically-induced flow field disturbs the boundary layer and enhances the convective heat transfer coefficient. However, the enhancement level is more remarkable in natural convection. In this study, the influence of a corona jet on heat transfer in rectangular channels with flat and longitudinal electrodes will be studied. The emitting and collecting electrodes are metallic strips attached to opposite walls of the channel and are extended along the axis of the channel. The electric field governing equations are solved numerically using finite-volume method and the third-order QUICK scheme is utilized for discretization of the charge fluxes. The distribution of electric field and charge density on the cross section of the channel is obtained to find the electric body-force at each point. In the presence of electric and buoyancy forces, the momentum and energy equations are solved to determine the level of enhancement of convective heat transfer using corona discharge.
An experimental study was performed to qualitatively investigate the effect of an applied electri... more An experimental study was performed to qualitatively investigate the effect of an applied electric field on two-phase flow patterns in convective flow boiling of R-134a inside a channel. The experiments were performed in a horizontal channel with 3 mm hydraulic diameter. Inlet vapor qualities of 5%, 40%, and 80% at a flow Reynolds number of 1000 and heat flux of 20 kW/m2 are reported here. The flow regimes were analyzed by a high-speed digital camera. The camera could provide a frames speed of up to 3025 frames/s. The electric field was maintained between the upper and lower walls of the channel. The current visualization study suggests some definitive characteristics of the flow regime and the mechanism behind the EHD-enhanced convective boiling in a narrowly-spaced channel. It is found that at the lower range of applied potential, the flow patterns change from stratified wavy flow to a plug flow type with large slugs spanning the height of the channel. At the higher range of applied potential, a new flow regime took place. This flow pattern was associated with the cross motion of the liquid inside the channel. One reason for heat transfer enhancement can be attributed to the strong heat/momentum transfer between the vapor slugs and liquid cross motions.
Microchannels have been widely used by the refrigeration/air conditioning industry. Zhao et al. (... more Microchannels have been widely used by the refrigeration/air conditioning industry. Zhao et al. (2000a,b) presented their experimental results for CO2 flow boiling in microchannels. In the present study, empirical modeling is performed to correlate the experimental data. Five state-of-the-art, large-tube empirical models are compared with the experimental data. It is found that these correlations fail to satisfactorily predict the experimental data for microchannels. This indicates that the heat transfer behavior in microchannels is different from that in large channels. A new correlation is developed based on Liu and Winterton’s correlation by introducing confinement factor to account for the micro-scale effects. The new model agrees well with the experimental results for CO2 flow boiling in microchannels within ±20%.
Heat Transfer Engineering, 2006
Corona discharge in air was used to generate a secondary flow that significantly improved the con... more Corona discharge in air was used to generate a secondary flow that significantly improved the convective heat transfer coefficient in the developing region of a square duct. A wire electrode, charged with a high voltage, was placed at the center of the duct. The electrode ionized the air and created a corona current, which was measured experimentally and used in
International Journal of Engineering, Nov 1, 1988
International Journal of Heat and Mass Transfer, 1986
Experiments were conducted to determine the axial distribution of per-facet and per-cycle heat tr... more Experiments were conducted to determine the axial distribution of per-facet and per-cycle heat transfer coefficients for turbulent flow in an isothermal, corrugated-wall duct with variable interwall spacing. The investigation was conducted via a mass transfer technique. The focus of the ...
Heat Transfer Engineering, Apr 1, 2007
The predication of nonlinear response of steel buildings under elevated temperatures from fire lo... more The predication of nonlinear response of steel buildings under elevated temperatures from fire loading can be achieved by sequentially using heat-transfer analysis and stress analysis. A combined computational fluid dynamics (CFD) and structural stress ...
Science And Technology For The Built Environment, Jul 1, 2002
EHD-enhanced condensation of alternative refrigerants in smooth and corrugated tubes. A Gidwani, ... more EHD-enhanced condensation of alternative refrigerants in smooth and corrugated tubes. A Gidwani, M Molki, MM Ohadi HVAC & R Research 8:33, 219-237, 7/2002. An experimental study of EHD-enhanced in-tube condensation of alternative refrigerants is presented. ...