M. Bahrami
Simon Fraser University, Mechatronics System Engineering, Faculty Member
Convective heat transfer of laminar, single-phase flow in rough microtubes is studied. Wall roughness and slope are assumed to possess Gaussian, isotropic distributions. Fractal concepts are used to model the rough microtube. It is shown... more
Convective heat transfer of laminar, single-phase flow in rough microtubes is studied. Wall roughness and slope are assumed to possess Gaussian, isotropic distributions. Fractal concepts are used to model the rough microtube. It is shown that due to the existence of wall roughness, both cross-sectional and inside surface areas are increased. A new concept is defined as a figure of merit for assessing thermal performance of rough microtubes. As a result of increasing roughness, an enhancement is observed in the thermal performance of microtubes. The present model can be extended to analyze other geometries such as rectangular and trapezoidal microchannels.
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Pressure drop through micro-pillar-integrated mini/microchannels is studied experimentally and analytically. Following our previous studies, the low aspect ratio micropillars embedded in a microchannel are modeled as a porous medium... more
Pressure drop through micro-pillar-integrated mini/microchannels is studied experimentally and analytically. Following our previous studies, the low aspect ratio micropillars embedded in a microchannel are modeled as a porous medium sandwiched between channel walls. The pressure drop is expressed as a function of the salient geometrical parameters such as channel dimension, diameter and spacing between the adjacent cylinders as well as their arrangement. To verify the developed model, several silicon/glass samples with and without integrated pillars are fabricated using the deep reacting ion etching (DRIE) technique. Pressure drop measurements are performed over a range of water flow rates ranging from 0.1 ml/min to 0.5 ml/min. The proposed model is successfully verified with the present experimental data. A parametric study is performed by employing the proposed model, which shows that the flow resistance has a reverse relationship with the micro-pillar diameter and the mini/microc...
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Fundamental problem of heat transfer within a half-space due to a moving heat source of hyperelliptical geometry is studied in this work. The considered hyperelliptical geometry family covers a wide range of heat source shapes, including... more
Fundamental problem of heat transfer within a half-space due to a moving heat source of hyperelliptical geometry is studied in this work. The considered hyperelliptical geometry family covers a wide range of heat source shapes, including star-shaped, rhombic, elliptical, rectangular with round corners, rectangular, circular, and square. The effects of the heat source speed, aspect ratio, corners, and orientation are investigated using the general solution of a moving point source on a half-space and superposition. Selecting the square root of the heat source area as the characteristics length scale, it is shown that the maximum temperature within the half-space is a function of the heat source speed (Peclet number) and its aspect ratio. It is observed that the details of the exact heat source shape have negligible effect on the maximum temperature within the half-space. New general compact relationships are introduced that can predict the maximum temperature within the half-space wi...
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Polymer electrolyte membrane fuel cells (PEMFC) efficiently convert the reaction energy of hydrogen and oxygen to electricity, water and heat. The oxygen reduction reaction occurs in composite nanostructured catalyst layers (CL) formed... more
Polymer electrolyte membrane fuel cells (PEMFC) efficiently convert the reaction energy of hydrogen and oxygen to electricity, water and heat. The oxygen reduction reaction occurs in composite nanostructured catalyst layers (CL) formed from Pt nanoparticles supported on a network of carbon particle agglomerates. Oxygen reaches the reaction site through diffusion. Understanding the diffusion properties of CL is vital to proper design and operation of CL and PEMFC. Measuring the diffusivity of thin porous layers is challenging, as is selecting a suitable substrate and appropriate CL coating procedures. In this work, CL is coated on 70 μm thick hydrophobic porous polymer substrates with a Mayer bar coater. Several samples are prepared and their thickness are measured accurately. The diffusivity of the CL and the substrate are measured using a dry diffusivity test bed and the resulting CL-diffusivity values are determined for different Pt loadings.
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Polymer electrolyte membrane fuel cells (PEMFC) efficiently convert the reaction energy of hydrogen and oxygen to electricity, water and heat. The oxygen reduction reaction occurs in composite nanostructured catalyst layers (CL) formed... more
Polymer electrolyte membrane fuel cells (PEMFC) efficiently convert the reaction energy of hydrogen and oxygen to electricity, water and heat. The oxygen reduction reaction occurs in composite nanostructured catalyst layers (CL) formed from Pt nanoparticles supported on a network of carbon particle agglomerates. Oxygen reaches the reaction site through diffusion. Understanding the diffusion properties of CL is vital to proper design and operation of CL and PEMFC. Measuring the diffusivity of thin porous layers is challenging, as is selecting a suitable substrate and appropriate CL coating procedures. In this work, CL is coated on 70 μm thick hydrophobic porous polymer substrates with a Mayer bar coater. Several samples are prepared and their thickness are measured accurately. The diffusivity of the CL and the substrate are measured using a dry diffusivity test bed and the resulting CL-diffusivity values are determined for different Pt loadings.
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Integrating the cooling systems of power electronics and electric machines (PEEMs) with other existing vehicle thermal management systems is an innovative technology for the next-generation hybrid electric vehicles (HEVs). As such, the... more
Integrating the cooling systems of power electronics and electric machines (PEEMs) with other existing vehicle thermal management systems is an innovative technology for the next-generation hybrid electric vehicles (HEVs). As such, the reliability of PEEM must be assured under different dynamic duty cycles. Accumulation of excessive heat within the multilayered packages of PEEMs, due to the thermal contact resistance between the layers and variable temperature of the coolant, is the main challenge that needs to be addressed over a transient thermal duty cycle. Accordingly, a new analytical model is developed to predict transient heat diffusion inside multilayered composite packages. It is assumed that the composite exchanges heat via convection and radiation mechanisms with the surrounding fluid whose temperature varies arbitrarily over time (thermal duty cycle). As such, a time-dependent conjugate convection and radiation heat transfer is considered for the outer-surface. Moreover,...
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Integrating the cooling systems of power electronics and electric machines (PEEMs) with other existing vehicle thermal management systems is an innovative technology for the next-generation hybrid electric vehicles (HEVs). As such, the... more
Integrating the cooling systems of power electronics and electric machines (PEEMs) with other existing vehicle thermal management systems is an innovative technology for the next-generation hybrid electric vehicles (HEVs). As such, the reliability of PEEM must be assured under different dynamic duty cycles. Accumulation of excessive heat within the multilayered packages of PEEMs, due to the thermal contact resistance between the layers and variable temperature of the coolant, is the main challenge that needs to be addressed over a transient thermal duty cycle. Accordingly, a new analytical model is developed to predict transient heat diffusion inside multilayered composite packages. It is assumed that the composite exchanges heat via convection and radiation mechanisms with the surrounding fluid whose temperature varies arbitrarily over time (thermal duty cycle). As such, a time-dependent conjugate convection and radiation heat transfer is considered for the outer-surface. Moreover,...
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Research Interests: Mechanical Engineering, Applied Mathematics, Microstructure, Microfluidics, Heat Transfer, and 15 moreBoundary Conditions, Laminar Flow, Heat Flux, Microchannel, Cross Section, Length scale, Moment of Inertia, Nusselt Number, Boundary Condition, Numerical Solution, Chemical Engineering Heat & Mass Transfer, Exact solution methods, Parametric Study, Compact Model, and cross sectional area
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ABSTRACT A novel distributed transient thermal model is proposed to investigate the thermal behavior of nickel-metal hydride (NiMH) batteries under fast-charging processes at constant currents. Based on the method of integral... more
ABSTRACT A novel distributed transient thermal model is proposed to investigate the thermal behavior of nickel-metal hydride (NiMH) batteries under fast-charging processes at constant currents. Based on the method of integral transformation, a series-form solution for the temperature field inside the battery core is obtained that takes account for orthotropic heat conduction, transient heat generation, and convective heat dissipation at surfaces of the battery. The accuracy of the developed theoretical model is confirmed through comparisons with numerical and experimental data for a sample 30 ampere-hour NiMH battery. The comparisons show that even the first term of the series solution fairly predicts the temperature field with the modest numerical cost. The thermal model is also employed to define an efficiency for charging processes. Our calculations confirm that the charging efficiency decreases as the charging current increases.
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This paper outlines a novel approximate model for determining the pressure drop of laminar, single-phase flow in slowly-varying microchannels of arbitrary cross-section based on the solution of a channel of elliptical cross-section. A new... more
This paper outlines a novel approximate model for determining the pressure drop of laminar, single-phase flow in slowly-varying microchannels of arbitrary cross-section based on the solution of a channel of elliptical cross-section. A new nondimensional parameter is introduced as a criterion to identify the significance of frictional and inertial effects. This criterion is a function of the Reynolds number and geometrical parameters of the cross-section; i.e., perimeter, area, cross-sectional polar moment of inertia, and channel length. It is shown that for the general case of arbitrary cross-section, the cross-sectional perimeter is a more suitable length scale. An experimental investigation is conducted to verify the present model; 5sets of rectangular microchannels with converging–diverging linear wall profiles are fabricated and tested. The collected pressure drop data are shown to be in good agreement with the proposed model. Furthermore, the presented model is compared with the numerical and experimental data available in the literature for a hyperbolic contraction with rectangular cross-section.
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A compact analytical model is developed for predicting thermal contact resistance (TCR) of nonconforming rough contacts of bare solids in a vacuum. Instead of using probability relationships to model the size and number of microcontacts... more
A compact analytical model is developed for predicting thermal contact resistance (TCR) of nonconforming rough contacts of bare solids in a vacuum. Instead of using probability relationships to model the size and number of microcontacts of Gaussian surfaces, a novel approach is taken by employing the “scale analysis method.” It is demonstrated that the geometry of heat sources on a half-space for microcontacts is justifiable for an applicable range of contact pressure. It is shown that the surface curvature and contact pressure distribution have no effect on the effective microthermal resistance. The present model allows TCR to be predicted over the entire range of nonconforming rough contacts from conforming rough to smooth Hertzian contacts. A new nondimensional parameter, i.e., ratio of the macro- over microthermal resistances, is introduced as a criterion to identify three regions of TCR. The present model is compared to collected TCR data for SS304 and showed excellent agreemen...
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In an adsorption chiller, the refrigerant (water) operating pressure is low (0.5e5 kPa) and the cooling power generation of a flooded evaporator is affected by the height of water column. To resolve this issue, we experimentally... more
In an adsorption chiller, the refrigerant (water) operating pressure is low (0.5e5 kPa) and the cooling power generation of a flooded evaporator is affected by the height of water column. To resolve this issue, we experimentally investigate the performance of a flooded evaporator as a function of water height. The results show an optimum water height equal to 80% of the tube diameter leading to achieve the highest cooling power. Under this condition, the internal and external thermal resistances on the inside and outside of the evaporator tubes account for up to 73% of the overall thermal resistance. To reduce the internal thermal resistance, twisted and Z-type turbulent flow generators are incorporated into the evaporator tubes. The evaporator cooling power shows an increase by 12% and 58% when twisted tape and Z-type turbulators are used at a cost of an increase in the internal pressure drop by 2.5 and 14.5 times, respectively. The twisted tape and Z-type turbulators improve the average specific cooling power of the adsorption chiller by 9% and 47%, respectively. To reduce the external thermal resistance, the outside surface of the evaporator tubes is coated with porous copper. The coated evaporator increases the overall heat transfer coefficient by 1.4 times and improves the specific cooling power of the adsorption chiller by 48% compared to the uncoated tubes.