In this paper, the mixed natural-forced convection is experimentally investigated for the heat transfer oil-copper oxide (HTO-CuO) nanofluid flow upward in a vertical tube. The flow regime is laminar and the temperature of the tube surface is constant. The effect of the nanoparticles concentration on the heat transfer rate and the pressure drop is studied as Richardson number varies between 0.1 and 0.7. It is observed that the mixed convection heat transfer rate increases with both the nanoparticles concentration and Richardson number. New correlations are proposed to predict the Nusselt number of the nanofluid flow with the reasonable accuracy. As the heat transfer enhancement methods usually accompany with increment in the pressure drop, the figure of merit is evaluated experimentally. As such the maximum figure of merit of 1.31 is achieved using the 1.5% concentration of the nanoparticles in Richardson number of 0.7. This study provides a platform to design next generation of low flow rate nanofluid-based heat exchangers and may improve the accuracy of predicting the mixed convection characteristics of nanofluid flows.

The contradictory results in literature regarding water-based carbon nanofluids for the turbulent regime are addressed in this paper. Convective heat transfer and friction factor of MWCNT-water nanofluids inside a vertical circular tube are investigated experimentally. The tested tubes are under constant heat flux boundary condition and the flow regime inside the tubes is turbulent. Distilled water and nanofluids with different particle weight concentrations of 0.05%, 0.1%, and 0.2% are utilized as the working fluids. Such tube and flow conditions are considered to specifically answer the question "whether water-based carbon, nanofluids increase the heat transfer rate or friction factor in the turbulent regime". The required thermo-physical properties of the MWCNT water nanofluids for calculating the convective heat transfer coefficient and pressure drop are measured. Observations clearly show that the heat transfer coefficient of nanofluid is higher than that of the base fluid and increases with the particle concentrations. Moreover, the measurements show that the pressure drop of nanofluid is slightly higher than that of the base fluid and does not increase significantly with increasing the nanoparticles volume fraction. Also, two correlations are proposed using the experimental data to predict the Nusselt number and friction factor of the nanofluid flow inside vertical tubes. Finally, the heat transfer performance analysis of MWCNT-water nanofluids show that the thermal performance factor for all cases are greater than unity which indicate that this nanofluid enhances the heat transfer without huge penalty in pumping power. (C) 2016 Elsevier Ltd. All rights reserved.

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, arbitrary heat generation inside the layers and thermal contact resistances between the layers are taken into account. New closed-form relationships are developed to calculate the temperature distribution inside multilayered media. The present model is used to find an optimum value for the angular frequency of the surrounding fluid temperature to maximize the interfacial heat flux of composite media; up to 10% higher interfacial heat dissipation rate compared to constant fluid-temperature case. An independent numerical simulation is also performed using COMSOL Multiphysics; the maximum relative difference between the obtained numerical data and the analytical model is less than 6%.

An experimental study is carried out on heat transfer characteristics of a nano-refrigerant flow during condensation inside a horizontal smooth tube. Experiments are conducted for three different working fluid types including: (i) pure refrigerant (R600a); (ii) refrigerant/lubricant (R600a/oil); and (iii) nanorefrigerant: refrigerantflubricant/nanoparticles (R600a1 oil/CuO). Polyolester oil (POE) is utilized as the lubricant in the two latter cases. In addition, nano-refrigerants (R600a/oil/CuO) are prepared by dispersing CuO nanoparticles with different mass fractions of 0.5%, 1% and 1.5% in the baseline mixture (R600a/oil). The implemented experiments covered a wide range of variables including: (i) mass fluxes from 154.8 to 265.4 kg/m(2)/s; (ii) vapor qualities between 10% and 80%; (iii) heat flux from 17 to 20 kW/m(2); and (iv) condensation pressure from 5.1 to 6.2 bar. It is shown that significant heat transfer enhancement is achieved by adding nanoparticles to the baseline mixture and pure refrigerant. The maximum heat transfer augmentation was observed for nano-refrigerant with 1.5% mass fraction; 83% higher heat transfer rate compared to pure-refrigerant fluid flow at the same experimental conditions. (C) 2015 Elsevier Inc. All rights reserved.

Smart dynamic thermal management (SDTM) is a key enabling technology for optimal design of the emerging transient heat exchangers/heat sinks associated with advanced power electronics and electric machines (APEEM). The cooling systems of APEEM undergo substantial transition as a result of time-varying thermal load over a duty cycle. Optimal design criteria for such dynamic cooling systems should be achieved through addressing internal forced convection under time-dependent heat fluxes. Accordingly, an experimental study is carried out to investigate the thermal characteristics of a laminar fully-developed tube flow under time-varying heat fluxes. Three different transient scenarios are implemented under: (i) step; (ii) sinusoidal; and (iii) square-wave time-varying thermal loads. Based on the transient energy balance, exact closed-form relationships are proposed to predict the coolant bulk temperature over time for the aforementioned scenarios. In addition, based on the obtained experimental data and the methodology presented in Fakoor-Pakdaman et al. (2014), semi-analytical relationships are developed to calculate: (i) tube wall temperature; and (ii) the Nusselt number over the implemented duty cycles. It is shown that there is a ‘cut-off’ angular frequency for the imposed power beyond which the heat transfer does not feel the fluctuations. The results of this study provide the platform for temperature-aware dynamic cooling solutions based on the instantaneous thermal load over a duty cycle.

Developing next generation transient heat exchangers is a transformative technology for efficient ther- mal management of advanced power electronics and electric machines (APEEM) inside hybrid electric, electric, and fuel cell vehicles as well as renewable energies (wind, solar, tidal). Optimal design criteria for such dynamic heat exchangers should be achieved through addressing internal forced convection with arbitrary flow unsteadiness under dynamic time dependent thermal loads. Exact analytical solutions were obtained for laminar forced-convective heat transfer under arbitrary time-dependent heat flux for steady flow inside a tube (Fakoor-Pakdaman et al., 2014). In this study, the energy equation is solved ana- lytically for arbitrary unsteady flow between parallel plates under dynamic time and position dependent heat flux. As such, laminar pulsating flow between two parallel plates is considered under a harmonic wall heat flux. Exact relationships are obtained to find; (i) temperature distribution for the fluid; (ii) fluid bulk temperature; and (iii) the local and time averaged Nusselt number. New compact relationships are proposed to find the thermal entrance length and the cyclic fully-developed Nusselt number. It is shown that the period of the Nusselt number oscillations is the least common multiple of the periods of the imposed harmonic heat flux and the pulsating flow. We obtained a relationship for the ‘cut-off’ angular frequency of pulsating flow beyond which the heat transfer does not feel the pulsation. This study also shows that for a given harmonic wall heat flux, there is an optimal pulsating flow velocity, with optimum frequency, which enhances the time averaged Nusselt number by up to 27%. 2014 Elsevier Ltd. All rights reserved.

An empirical investigation is performed on boiling heat transfer characteristics of R600a refrigerant flow inside horizontal flattened tubes. Round copper tubes of 8.7 mm inner diameter are deformed into flattened shapes with different internal heights of 6.9, 5.5, and 3.4mmas the test sections. Effects of different parameters such as mass flux, vapor quality, and internal tube height on the heat transfer coefficient are studied. It is shown that flattening the tube causes significant heat transfer enhancement. The maximum augmentation ratio of 163% is obtained for the flattened tube with the maximum aspect ratio of 3.56. A new correlation is developed based on the obtained experimental data to predict the heat transfer coefficient in flattened tubes; 90% of the acquired data are predicted within. Consequently, the tested flattened tube with maximum heat transfer rate is selected to study the effects of nanoparticles. Experiments are conducted for three different working fluid types including 1) pure refrigerant (R600a); 2) refrigerant/ lubricant (R600a/oil); and 3) nanorefrigerant: refrigerant/lubricant/nanoparticles (R600a/oil/CuO). Polyolester oil is used as the lubricant in the two latter cases. In addition, nanorefrigerants (R600a/oil/CuO) are prepared by dispersing CuO nanoparticles with different mass fractions of 0.5, 1, and 1.5% in the baseline mixture (R600a/oil). Thereby, maximum heat transfer enhancement of79% is achieved compared to the pure refrigerant case. The results of this study provide the platform for designing compact efficient heat exchangers/heat sinks for the emerging thermal engineering applications under high thermal load conditions.

An experimental investigation is carried out to study the condensation and pressure drop characteristics of R-600a inside a helical tube-in-tube heat exchanger. The axis of the helical condenser was located at inclination angles of zero, +30 degrees, +60 degrees and +90 degrees from horizontal. The test condenser was a counter flow helical tube-in-tube heat exchanger. The diameter, pitch, height and the number of coil turns were 305 mm, 35 mm, 210 mm and 6, respectively. Experiments were also implemented in a straight tube-in-tube condenser; all tests were performed at average saturation temperatures ranging between 38.5 and 47 degrees C. Refrigerant mass fluxes varied in the range of 155-265.5 kg m(-2) S. The average vapor quality varied between 0.11 and 0.78. The effects of inclination angle, mass flux and average vapor quality on the heat transfer coefficient and pressure drop are discussed. It is shown that the highest and lowest values of the heat transfer coefficient occur for 30 degrees and 90 degrees inclination angles respectively, while the pressure drop has the maximum value for the horizontal case. Compared to the horizontal straight condenser, the average heat transfer coefficient and the pressure drop of the horizontal helical condenser increase in the range of 24-165% and 33-157%, respectively. Moreover, the performance index values of horizontal helical condenser are about 15-41% higher than that of inclination angle of 90 degrees. (C) 2015 Elsevier Ltd. All rights reserved.

Heatsinks are essential parts of any thermal management system. High performance heatsinks are required for the cooling systems to be able to manage the ever-increasing power density in electronics and power electronics. The focus of this paper is on the design of high performance naturally-cooled heatsinks with vertical rectangular interrupted fins. A systematic analytical approach is taken, to solve the governing equations of the air flow and heat transfer. Closed-form correlations are presented for temperature and velocity distribution, and an easy-to-use method is introduced to design such heatsinks. Numerical simulations are used to provide better understanding of the physics of flow and heat transfer mechanism. An extensive experimental study is also conducted to verify the results from analytical solution and numerical simulation. Results show that the new-designed heatsinks are capable of dissipating heat up to 5 times more than currently available naturally-cooled heatsinks, with up to 30% less weight. The new heatsinks can increase the capacity of passive thermal management systems significantly.

Developing next generation transient heat exchangers is a transformative technology for efficient thermal management of advanced power electronics and electric machines (APEEM) inside hybrid electric, electric, and fuel cell vehicles as well as renewable energies (wind, solar, tidal). Optimal design criteria for such dynamic heat exchangers should be achieved through addressing internal forced convection with arbitrary flow unsteadiness under dynamic time dependent thermal loads. Exact analytical solutions were obtained for laminar forced-convective heat transfer under arbitrary time-dependent heat flux for steady flow inside a tube (Fakoor-Pakdaman et al., 2014). In this study, the energy equation is solved analytically for arbitrary unsteady flow between parallel plates under dynamic time and position dependent heat flux. As such, laminar pulsating flow between two parallel plates is considered under a harmonic wall heat flux. Exact relationships are obtained to find; (i) temperature distribution for the fluid; (ii) fluid bulk temperature; and (iii) the local and time averaged Nusselt number. New compact relationships are proposed to find the thermal entrance length and the cyclic fully-developed Nusselt number. It is shown that the period of the Nusselt number oscillations is the least common multiple of the periods of the imposed harmonic heat flux and the pulsating flow. We obtained a relationship for the ‘cut-off’ angular frequency of pulsating flow beyond which the heat transfer does not feel the pulsation. This study also shows that for a given harmonic wall heat flux, there is an optimal pulsating flow velocity, with optimum frequency, which enhances the time averaged Nusselt number by up to 27%.

A new all-time analytical model is developed to predict transient internal forced-convection heat transfer under arbitrary time-dependent wall temperature. Slug flow condition is assumed for the velocity profile inside the tube. The solution to the time-dependent energy equation for a step wall temperature is generalized for arbitrary time variations in surface temperature using Duhamel's theorem. A harmonic boundary temperature is considered, and new compact closed-form relationships are proposed to predic: 1)fluid temperature distribution; 2)fluid bulk temperature; 3)wall heat flux; and 4)the Nusselt number. An optimum value is found for the dimensionless angular frequency of the wall temperature to maximize the heat transfer rate of the studied unsteady forced-convective process. Such dimensionless parameter depends upon the imposed-temperature angular frequency, fluid thermophysical properties, and tube geometrical parameters. A general surface temperature is considered, and the temperature field inside the medium is obtained using a superposition technique. An independent numerical simulation is performed using ANSYS (R) Fluent. The comparison between the obtained numerical data and the present analytical model shows good agreement: a maximum relative difference less than 4.9%.

Recently developed graphite plates with large in-plane thermal conductivity are considered as promising alternative to conventional metallic plate heat exchangers (PHE). A new analytical model is developed to study the impact, and the potentials, of the emerging orthotropic graphite-based plates in PHE under various convective regimes. Closed-form relationships are obtained for temperature and heat flux distributions, and applied to perform a comprehensive parametric study on the orthotropic conductivity effects. Our results show that increasing the in-plane thermal conductivity leads to significant changes in heat flow pattern and reduction in temperature variation along the plate. In spite of the remarkable effects of in-plane thermal conductivity on the heat flow pattern, through-plane thermal conductivity plays the key role in controlling the total heat transfer between the hot and cold fluid streams through the plate. Moreover, a new critical through-plane conductivity is proposed to calculate the maximum value of thermal conductivity that provides the highest heat transfer rate through orthotropic slabs. The critical value also includes convective heat transfer resistance of the fluid side and the plate thickness effects. To verify the present model, an independent numerical study is conducted using COMSOL Multiphysics. The analytical results are compared with the obtained numerical data as well as an existing data set in the literature and show a great agreement with less than 5% relative difference.

In most engineering applications, e.g., hybrid electric vehicles, the multilayered electronic packages generate arbitrary heat over a transient thermal duty cycle. In addition, the outer surface of such media endures time-dependent temperature as a result of variable coolant temperature during driving/duty cycles. As such, a new analytical model is developed to predict transient heat conduction inside multilayered composite media with arbitrary heat generation inside the layers. It is assumed that the temperature of the outer surface varies periodically over time. New compact closed-form relationships are developed for calculating 1) the temperature distribution inside multilayered media, 2) the average temperature of each layer, and 3) the interfacial heat flux. As an example, the methodology is applied to a two-concentric-cylinder composite. A detailed parametric study is conducted, and the critical values for the dimensionless parameters are evaluated; beyond these values, the temperature field inside the media is not affected considerably for any combination of other variables. It is shown that there is an optimum angular frequency that maximizes the amplitude of the interfacial heat flux. An independent numerical simulation is also performed using commercially available software ANSYS; the maximum relative difference between the obtained numerical data and the analytical model is less than 2%.

A new closed-form analytical model is developed to predict transient laminar forced convection inside a circular tube following a time-wise step change in the wall heat flux. The proposed all-time model is based on a blending of two asymptotes; i) short-time asymptote: transient pure conduction in an infinite cylinder and ii) long-time asymptote: steady-state convective heat transfer inside a circular duct. Different fluid velocity profiles are taken into consideration and the model covers: i) Slug Flow (SF); ii) Hydrodynamically Fully Developed Flow (HFDF); and iii) Simultaneously Developing Flow (SDF) conditions. The present model is developed for the entire range of the Fourier and Prandtl numbers. As such, shortand long-time asymptotes for the fluid bulk temperature are obtained. The Nusselt number is defined based on the local temperature difference between the tube wall temperature and the fluid bulk temperature. It is shown that irrespective of the velocity profile, at the initial times the Nusselt number is only a function of time. However, at the steady state condition it depends solely upon the axial location. In addition, during the transient period, the Nusselt number is much higher than that of the long-time response. We also performed an independent numerical simulation using COMSOL Multiphysics to validate the present analytical model. The comparison between the numerical and the present analytical model shows good agreement; a maximum relative difference less than 9.1%.

A new all-time model is developed to predict transient laminar forced convection heat transfer inside a circular tube under arbitrary time-dependent heat flux. Slug flow condition is assumed for the velocity profile inside the tube. The solution to the time-dependent energy equation for a step heat flux boundary condition is generalized for arbitrary time variations in surface heat flux using a Duhamel’s integral technique. A cyclic time-dependent heat flux is considered and new compact closed-form relationships are proposed to predict: i) fluid temperature distribution inside the tube ii) fluid bulk temperature and iii) the Nusselt number. A new definition, cyclic fully-developed Nusselt number, is introduced and it is shown that in the thermally fully-developed region the Nusselt number is not a function of axial location, but it varies with time and the angular frequency of the imposed heat flux. Optimum conditions are found which maximize the heat transfer rate of the unsteady laminar forced-convective tube flow. We also performed an independent numerical simulation using ANSYS to validate the present analytical model. The comparison between the numerical and the present analytical model shows great agreement; a maximum relative difference less than 5.3%.

In this study, pressure drop characteristics of nanofluid flow inside vertical helically coiled tubes are investigated experimentally for the laminar flow regime. The temperature of the tube wall is maintained constant at around 95 degrees C to have isothermal boundary condition Experiments are implemented for fluid flow inside helically coiled tubes and a straight one. A wide range of various variables is taken into account. Pitch to tube-diameter ratio ranges between 1.6 and 6.1 and coil-to-tube diameter ratio varies from 14.1 to 20.5. Heat transfer oil is used as the base fluid, and Multi-Walled Carbon NanoTubes (MWCNTs) are utilized as the additive to provide the nanofluids. The working fluids are extremely temperature dependent, so rough correlations are proposed to predict their thermo-physical properties. Regarding the experimental data, utilization of helical coiled tubes instead of straight ones increases the pressure drop exponentially. Irrespective of the tube geometry in which the fluid flows, nanofluid flows show higher rate of pressure drop compared to that of the base fluid flow. Finally according to the observations, combination of the two techniques used in this study causes the pressure of the fluid flow to drop significantly along the test section. (C) 2012 Elsevier Masson SAS. All rights reserved.

This study deals with an experimental investigation on the thermo-physical properties and overall performance of MWCNT/heat transfer oil nanofluids flow inside vertical helically coiled tubes. Data were acquired for the laminar flow in the thermal entrance region, while the temperature of the tube wall was constant at around 95°C leading to isothermal boundary condition. Pure heat transfer oil and nanofluids with particle weight concentrations of 0.1%, 0.2% and 0.4% were utilized as the working fluids. It was observed that most conventional models fail to predict the thermo-physical properties of the applied nanofluids accurately, especially in case of the specific heat capacity. Therefore, rough empirical correlations were developed to estimate such properties for the working fluids. In addition, the overall performance of the tested helically coiled tubes was assessed based on the performance index and optimum work conditions were determined. High overall performance index of up to 6.4 was obtained for the simultaneous utilization of both heat transfer enhancement techniques considered in this paper. Hence, applying the methods studied here could be considered as a good choice in practical applications.

In this study, heat transfer enhancement of a nanofluid flow inside vertical helically coiled tubes has been investigated experimentally in the thermal entrance region. The temperature of the tube wall was kept constant at around 95°C to have isothermal boundary condition. Experiments were conducted for fluid flow inside straight and helical tubes. In these experiments, the effects of a wide range of different parameters such as Reynolds and Dean numbers, geometrical parameters and nanofluid weight fractions have been studied. In order to investigate the effect of the fluid type on the heat transfer, pure heat transfer oil and nanofluids with weight concentrations of 0.1, 0.2 and 0.4% were utilized as the working fluid. The thermo-physical properties of the working fluids were extremely temperature dependent; therefore, rough correlations were proposed to predict their properties. Based on the experimental data, utilizing helical coiled tubes instead of straight ones enhances the heat transfer rate remarkably. Besides, nanofluid flows showed much higher Nusselt numbers compared to the base fluid flow. Finally, it was observed that combination of the two enhancing methods has a noticeably high capability to the heat transfer rate.

Experiments are performed to investigate the single-phase flow heat transfer augmentation of MWCNT/HT-B oil in both smooth and microfinned helical tubes with constant wall temperature. The tests in laminar regime were carried out in helical tubes with three curvature ratios of 2R/d=25, 30 and 35. Flow Reynolds number varied from 170 to 1800 resulting in laminar flow regime. The effect of some parameters such as the nanoparticles concentration, the dimensionless curvature radius (2R/d) and the Reynolds number on heat transfer was investigated for the laminar flow regime. The weight fraction of nanoparticles in base fluid was less than 0.4%. within the applied range of Reynolds number; results indicated that for smooth helical tube the addition of nanoparticles to the base fluid enhanced heat transfer remarkably. However, compared to the smooth helical tube, the average heat transfer augmentation ratio due to nanoparticle addition for finned tube was small, about 17%. Also, by increasing the weight fraction of nanoparticles in microfinned helical tubes, no substantial changes were observed in the rate of heat transfer enhancement. For the pressure drop, the results show that the pressure drop of nanofluids was slightly higher than the base fluid and increase as the volume concentrations go up.