The effect of a synthetic jet on heat transfer rates in forced convection of water in micro-chann... more The effect of a synthetic jet on heat transfer rates in forced convection of water in micro-channels has been studied numerically. Unsteady computations of laminar flow have been performed for a twodimensional micro-channel, 200 µm high and 4.2 mm long, open at either end with the top surface hot and all other solid wall adiabatic. The orifice width was 50 µm, while the length of the orifice passage was 100 µm. The diaphragm was assumed to oscillate sinusoidally in time and parabolically in space. The effectiveness of the proposed cooling procedure has been evaluated by comparing the heat transfer rates with and without the synthetic jet. A maximum heat transfer enhancement of approximately 125% was achieved.
The differential Reynolds stress (RS) transport model for turbulent flow is used to study recircu... more The differential Reynolds stress (RS) transport model for turbulent flow is used to study recirculating flow (driven by a moving wall) in a rectangular cavity. Empirical cavity jet boundary conditions based on wall-jet and cavity-jet data are obtained. Good mean velocity field predictions are obtained with both the k-epsilon and RS-epsilon models. The stabilizing effect of flow curvature is well
A new method for the solution of the vector potentialvorticity formulation of the equations of a... more A new method for the solution of the vector potentialvorticity formulation of the equations of a fluid motion is presented in this paper. The fully coupled finite difference approximations to these equations have been solved using a general block tri-diagonal scheme. New ...
International Journal of Numerical Methods for Heat & Fluid Flow, 2007
Purpose – The development of novel cooling techniques is needed in order to be able to substantia... more Purpose – The development of novel cooling techniques is needed in order to be able to substantially increase the performance of integrated electronic circuits whose operations are limited by the maximum allowable temperature. Air cooled micro-channels etched in the silicon substrate have the potential to remove heat directly from the chip. For reasonable pressure drops, the flow in micro-channels is inherently laminar, so that the heat transfer is not very large. A synthetic jet may be used to improve mixing, thereby considerably increasing heat transfer. This paper seeks to address this issue. Design/methodology/approach – CFD has been used to study the flow and thermal fields in forced convection in a two-dimensional micro-channel with an inbuilt synthetic jet actuator. The unsteady Navier-Stokes and energy equations are solved. The effects of variation of the frequency of the jet at a fixed pressure difference between the ends of the channel and with a fixed jet Reynolds number, have been studied with air as the working fluid. Although the velocities are very low, the compressibility of air has to be taken into account. Findings – The use of a synthetic jet appreciably increases the rate of heat transfer. However, in the frequency range studied, whilst there are significant changes in the details of the flow, due primarily to large phase changes with frequency, there is little effect of the frequency on the overall rate heat transfer. The rates of heat transfer are not sufficiently large for air to be a useful cooling medium for the anticipated very large heat transfer rates in future generations of microchips. Research limitations/implications – The study is limited to two-dimensional flows so that the effect of other walls is not considered. Practical implications – It does not seem likely that air flowing in channels etched in the substrate of integrated circuits can be successfully used to cool future, much more powerful microchips, despite a significant increase in the heat transfer caused by synthetic jet actuators. Originality/value – CFD is used to determine the thermal performance of air flowing in micro-channels with and without synthetic jet actuators as a means of cooling microchips. It has been demonstrated that synthetic jets significantly increase the rate of heat transfer in the micro-channel, but that changing the frequency with the same resulting jet Reynolds number does not have an effect on the overall rate of heat transfer. The significant effect of compressibility on the phase shifts and more importantly on the apparently anomalous heat transfer from the “cold” air to the “hot” wall is also demonstrated.
ABSTRACT A numerical model has been developed to simulate heat and mass transfer phenomena during... more ABSTRACT A numerical model has been developed to simulate heat and mass transfer phenomena during a steam air sterilisation cycle of an operating industrial autoclave. The objective of this paper is to present a practical methodology applicable for evaluating transient heat and mass transfer phenomena by assuming quasisteady state conditions. As a first step, an empty autoclave was considered. The developed model utilises measured steam flow rates as input parameters and predicts the transient temperature and pressure profiles. The numerical results are validated against two measured on-site data sets, one obtained in summer and the other in winter. The detailed numerical results allow the major heat losses within the process were identified to be determined, thus presenting the importance of having such a numerical model when conducting analyses for evaluating potential thermal energy savings in thermal processes. It is shown, that in any cyclic operation involving the heating and cooling of the containing walls, the insulation should be placed on the heated side.
Solid State Ionics - Trends in the New Millennium - Proceedings of the 8th Asian Conference, 2002
This paper presents the effects of 6KV DC and AC electric fields on heat transfer by using vertic... more This paper presents the effects of 6KV DC and AC electric fields on heat transfer by using vertical heating surface in Freon 11 (R11) used as a dielectric liquid. The vertical heating surface consists of 17mm diameter 80mm long electrically heated brass tube surrounded by eight brass electrodes 1.8 mm in diameter. The heater was placed such that heating surface
Computational fluid dynamics can play an important role in the design of microelectromechanical s... more Computational fluid dynamics can play an important role in the design of microelectromechanical systems (MEMS)-based actuators, allowing an investigation of the underlying physical behavior of devices before proceeding to expensive manufacturing processes. Here, we present the results of a series of numerical simulations of synthetic jet actuators, so-called because they synthesize jets from the working fluid. The primary characteristic of these actuators is that over a cycle, the net mass flux is zero, yet the mean momentum flux is non-zero. The simulations consider both the internal and external flow-fields of a micro-scaled device, with the membrane motion specified using a simplified structural analysis. Key features of the flow in the external, orifice and cavity regions are presented and discussed. The actuator output is observed to vary linearly with a non-dimensional input parameter, and this may guide the design of flow control systems that use synthetic jet actuators. Finally, our fabrication strategy for a MEMS-based synthetic jet actuator is presented.
The effect of a synthetic jet on heat transfer rates in forced convection of water in micro-chann... more The effect of a synthetic jet on heat transfer rates in forced convection of water in micro-channels has been studied numerically. Unsteady computations of laminar flow have been performed for a twodimensional micro-channel, 200 µm high and 4.2 mm long, open at either end with the top surface hot and all other solid wall adiabatic. The orifice width was 50 µm, while the length of the orifice passage was 100 µm. The diaphragm was assumed to oscillate sinusoidally in time and parabolically in space. The effectiveness of the proposed cooling procedure has been evaluated by comparing the heat transfer rates with and without the synthetic jet. A maximum heat transfer enhancement of approximately 125% was achieved.
The differential Reynolds stress (RS) transport model for turbulent flow is used to study recircu... more The differential Reynolds stress (RS) transport model for turbulent flow is used to study recirculating flow (driven by a moving wall) in a rectangular cavity. Empirical cavity jet boundary conditions based on wall-jet and cavity-jet data are obtained. Good mean velocity field predictions are obtained with both the k-epsilon and RS-epsilon models. The stabilizing effect of flow curvature is well
A new method for the solution of the vector potentialvorticity formulation of the equations of a... more A new method for the solution of the vector potentialvorticity formulation of the equations of a fluid motion is presented in this paper. The fully coupled finite difference approximations to these equations have been solved using a general block tri-diagonal scheme. New ...
International Journal of Numerical Methods for Heat & Fluid Flow, 2007
Purpose – The development of novel cooling techniques is needed in order to be able to substantia... more Purpose – The development of novel cooling techniques is needed in order to be able to substantially increase the performance of integrated electronic circuits whose operations are limited by the maximum allowable temperature. Air cooled micro-channels etched in the silicon substrate have the potential to remove heat directly from the chip. For reasonable pressure drops, the flow in micro-channels is inherently laminar, so that the heat transfer is not very large. A synthetic jet may be used to improve mixing, thereby considerably increasing heat transfer. This paper seeks to address this issue. Design/methodology/approach – CFD has been used to study the flow and thermal fields in forced convection in a two-dimensional micro-channel with an inbuilt synthetic jet actuator. The unsteady Navier-Stokes and energy equations are solved. The effects of variation of the frequency of the jet at a fixed pressure difference between the ends of the channel and with a fixed jet Reynolds number, have been studied with air as the working fluid. Although the velocities are very low, the compressibility of air has to be taken into account. Findings – The use of a synthetic jet appreciably increases the rate of heat transfer. However, in the frequency range studied, whilst there are significant changes in the details of the flow, due primarily to large phase changes with frequency, there is little effect of the frequency on the overall rate heat transfer. The rates of heat transfer are not sufficiently large for air to be a useful cooling medium for the anticipated very large heat transfer rates in future generations of microchips. Research limitations/implications – The study is limited to two-dimensional flows so that the effect of other walls is not considered. Practical implications – It does not seem likely that air flowing in channels etched in the substrate of integrated circuits can be successfully used to cool future, much more powerful microchips, despite a significant increase in the heat transfer caused by synthetic jet actuators. Originality/value – CFD is used to determine the thermal performance of air flowing in micro-channels with and without synthetic jet actuators as a means of cooling microchips. It has been demonstrated that synthetic jets significantly increase the rate of heat transfer in the micro-channel, but that changing the frequency with the same resulting jet Reynolds number does not have an effect on the overall rate of heat transfer. The significant effect of compressibility on the phase shifts and more importantly on the apparently anomalous heat transfer from the “cold” air to the “hot” wall is also demonstrated.
ABSTRACT A numerical model has been developed to simulate heat and mass transfer phenomena during... more ABSTRACT A numerical model has been developed to simulate heat and mass transfer phenomena during a steam air sterilisation cycle of an operating industrial autoclave. The objective of this paper is to present a practical methodology applicable for evaluating transient heat and mass transfer phenomena by assuming quasisteady state conditions. As a first step, an empty autoclave was considered. The developed model utilises measured steam flow rates as input parameters and predicts the transient temperature and pressure profiles. The numerical results are validated against two measured on-site data sets, one obtained in summer and the other in winter. The detailed numerical results allow the major heat losses within the process were identified to be determined, thus presenting the importance of having such a numerical model when conducting analyses for evaluating potential thermal energy savings in thermal processes. It is shown, that in any cyclic operation involving the heating and cooling of the containing walls, the insulation should be placed on the heated side.
Solid State Ionics - Trends in the New Millennium - Proceedings of the 8th Asian Conference, 2002
This paper presents the effects of 6KV DC and AC electric fields on heat transfer by using vertic... more This paper presents the effects of 6KV DC and AC electric fields on heat transfer by using vertical heating surface in Freon 11 (R11) used as a dielectric liquid. The vertical heating surface consists of 17mm diameter 80mm long electrically heated brass tube surrounded by eight brass electrodes 1.8 mm in diameter. The heater was placed such that heating surface
Computational fluid dynamics can play an important role in the design of microelectromechanical s... more Computational fluid dynamics can play an important role in the design of microelectromechanical systems (MEMS)-based actuators, allowing an investigation of the underlying physical behavior of devices before proceeding to expensive manufacturing processes. Here, we present the results of a series of numerical simulations of synthetic jet actuators, so-called because they synthesize jets from the working fluid. The primary characteristic of these actuators is that over a cycle, the net mass flux is zero, yet the mean momentum flux is non-zero. The simulations consider both the internal and external flow-fields of a micro-scaled device, with the membrane motion specified using a simplified structural analysis. Key features of the flow in the external, orifice and cavity regions are presented and discussed. The actuator output is observed to vary linearly with a non-dimensional input parameter, and this may guide the design of flow control systems that use synthetic jet actuators. Finally, our fabrication strategy for a MEMS-based synthetic jet actuator is presented.
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Papers by J. Reizes