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Abstract: We report on the results of investigations into the characteristics of an air–water isothermal two-phase flow in minichannels, that is, in capillary tubes with inner diameters of 1 mm, 2.4 mm, and 4.9 mm, also in capillary rectangular channels with an aspect ratio of 1 to 9. The directions of flow were vertical upward, horizontal and vertical downward. Based on the authors 15 years of fundamental research into the gas–liquid two-phase flows in circular tubes and rectangular channels, we summarized the characteristics of the flow phenomena in a minichannel with special attention on the flow patterns, the time varying holdup and the pressure loss. The effects of the tube diameters and aspect ratios of the channels on these flow parameters and the flow patterns were investigated. Also the correlations of the holdup and the frictional pressure drop were proposed. [Copyright &y& Elsevier]

Abstract: Three-dimensional numerical simulations of the laminar flow and heat transfer of water in silicon microchannels with non-circular cross-sections (trapezoidal and triangular) were performed. The finite volume method was used to discretize the governing equations. Numerical results were compared with experimental data available in the literature, and good agreements were achieved. The effects of the geometric parameters of the microchannels were investigated, and the variations of Nusselt number with Reynolds number were discussed from the field synergy principle. The simulation results indicate that when the Reynolds numbers are less than 100, the synergy between velocity and temperature gradient is much better than the case with Reynolds number larger than 100. There is an abrupt change in the intersection angle between velocity and temperature gradient around . In the low Reynolds number region the Nusselt number is almost proportional to the Reynolds number, while in the high Reynolds number region, the increasing trend of Nusselt number with Reynolds number is much more mildly, which showed the applicability of the field synergy principle. In addition, for the cases studied the fully developed Nusselt number for the microchannels simulated increases with the increasing Reynolds number, rather than a constant. [Copyright &y& Elsevier]

Abstract: Transient heat conduction in solid prismatic bars of constant cross-sectional area having uniform heat generation and unsteady momentum transport in infinitely long ducts of arbitrary but constant cross-sectional area are examined. In both cases the solutions are mathematically modeled using a transient Poisson equation. By means of scaling analysis a general asymptotic model is developed for an arbitrary non-circular cross-section. Further, by means of a novel characteristic length scale, the solutions for a number of fundamental shapes are shown to be weak functions of geometry. The proposed models can be used to predict the dimensionless mean flux at the wall and the area averaged temperature or velocity for the tube, annulus, channel and rectangle for which exact series solutions exist. Due to the asymptotic nature of the proposed models, it is shown that they are also applicable to other shapes at short and long times for which no solutions or data exist. The root mean square (RMS) error based on comparisons with exact results is between 2.2–7.6 percent for all data considered. [Copyright &y& Elsevier]

Corrugated tubes are extensively unitized in enhancing heat transfer within tubular heat exchange devices. The efficiency of these corrugated tubes is closely linked to the geometric characteristics of the surface corrugations. In this study, we conducted an extensive series of Large Eddy Simulations on a representative type of enhanced tube featuring periodic inward corrugations (PIC). Our goal was to compile a comprehensive and precise dataset of the thermal and hydraulic properties specific to this design. The key design parameters that define the corrugated patterns are corrugation height (H), corrugation width (W), and the length of the straight section (P). We systematically explored these parameters to understand their impact on heat transfer and fluid dynamics. Among these parameters, corrugation height (H) has been identified as the most influential factor affecting flow and heat transfer properties. As the corrugation height varies from 1/8D to 3/8D, we observed a substantial increase in the friction factor and Nusselt number by a factor of 209 and 5, respectively, at Re = 1000; while the critical Reynolds number decreased roughly from 800 to 100. These findings underscore the significant role of corrugation height in shaping the flow and heat transfer performance of PIC tubes. The other two parameters, corrugation width and straight section length, were also found to be crucial in determining the overall heat transfer coefficient and pressure drop. When Re = 1000, variations in W/D from 1 to 3 resulted in a 1.3-fold increase in both the f and the Nu. Furthermore, altering the P/D from 2 to 6 led to a 2.1-fold increase in f and a 2.2-fold increase in Nu. The results of our study suggest that when utilizing this type of corrugated tubes, careful consideration is required to strike an optimal balance between heat transfer efficiency and pressure loss. The large-sets LES results presented in this paper can serve as valuable inputs for optimizing the design of PIC tubes. [ABSTRACT FROM AUTHOR]

In a previous paper [1] it was shown a proposal for a discrete event simulation (DEVS) model of a thermal zone. A quantized state integrator of order one (QSS) was used to integrate the evolution of the room air temperature. In order to increase the computational speed, this paper presents how the multi-layered wall, 1D conduction heat flow dynamics could be modified to allow its use with a second order QSS2 quantized integrator. The almost forgotten successive transition state method (see U. Yoshimi et al. [2] ) is modified with changes in the hold function. Finally to validate the results, a test room has been employed to compare the results with EnergyPlus v7.1. Three cases have been studied: fixed T z o n e , floating T z o n e and floating T z o n e plus a random convective heat gain. [ABSTRACT FROM AUTHOR]

Abstract: Along with the developments of lattice-Boltzmann method in simulation of fluid flow, its thermal features should be also improved. An important problem which is encountered in many cases is the conjugate heat transfer or heat transfer at the fluid–solid interface. In this case, a strict technique is required to calculate the heat exchange between fluid and solid. Only a few works have been published regarding this phenomenon. In this paper, we have presented a model for analysis of the conjugate heat transfer. The model has been validated by two benchmarks: (I) Fluid flow and heat transfer in a backward-facing step channel with heated thick wall, and (II) Fluid flow and heat transfer in a channel with infinite number of heated obstacles mounted on the wall. The numerical tips concerned in the simulation of these problems by lattice-Boltzmann method have been also represented in the paper. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental investigation on the dryout during flow boiling of R245fa, R134a and R32 inside a 0.96mm diameter single circular microchannel. In the present tests, the channel is not electrically heated; instead, the flow boiling is achieved by means of a secondary fluid (water) resulting in a non-uniform distribution of the heat flux along the channel. Since the wall temperature is limited by the temperature of the secondary fluid, the onset of dryout is detected by means of the standard deviation of the temperature readings in the wall. The wall temperature in fact displays larger fluctuations in the zone where dryout occurs, which are related to the presence of the liquid film drying up at the wall with an oscillating process. These temperature fluctuations are detected by means of the standard deviation of the wall temperature. These fluctuations never appear during flow boiling at low vapor qualities; they also disappear in the post-dryout zone. Experimental values of dryout quality measured with the above method are reported in this paper for mass velocity ranging between 100 and 900kgm−2 s−1, during annular flow. The present data, which covers a wide range of reduced pressure (between 0.05 and 0.34), has been compared against some critical heat flux models available in the literature. Since in the present data the heat flux is not uniform along the channel, each dryout point is characterized by its own boiling story. The actual heat flux profile can be used when comparing with the model by Revellin and Thome . For other models, which are developed for uniformly heated microchannels, the predicted CHF is compared to the average experimental heat flux in the channel. [Copyright &y& Elsevier]

Abstract This paper discusses on the heat and fluid flow around two co-rotating cylinders in the tandem arrangement. The non-dimensional rotating speed (R.S) varies in the range of 0 ≤ R.S ≤ 4 and different non-dimensional gap spaces such as G/D = 1.5, 2.0, and 3.0 are considered between the cylinders. Computations are performed at the Reynolds number of 200 with constant Prandtl number of 7.0. It is demonstrated that rotating the cylinders deforms the recirculating regions of both upstream and downstream cylinders in which the rate of this deformation changes as a function of the R.S and G/D. On the other hand, co-rotating the cylinders shows some additional events such as the azimuthal displacement of the front stagnation points and development of the negative lift coefficient for both cylinders. It is found that the instabilities of the shear layer for both upstream and downstream cylinders are maximum at R.S = 1 and with increasing the R.S, the vortex shedding suppresses around the cylinders due to dominating the fluid rotating zone. Finally, it is revealed that at higher R.S values, a uniform Nusselt number distribution can be observed on both cylinders regardless of the gap space between the upstream and downstream cylinders. Highlights • Heat and fluid flow of both cylinders strongly depends on the RS and G/D. • Vortex strength of both cylinders against the RS is stronger for small G/D values. • At high RS values, a uniform Nusselt number distribution develops on the cylinders. • The RS = 1 is found as the critical RS at Re = 200. • Oscillations of the lift and drag coefficients become maximum at RS = 1. [ABSTRACT FROM AUTHOR]

Abstract: In a recent paper by Damseh et al. (Int. J. Thermal Sci. 48, 1658–1663, 2009), the title problem for an infinite vertical plate has been investigated numerically using the fourth-order Runge–Kutta method. In the present paper the full analytical solution is given. Several new features emerging from this approach are discussed in detail. [Copyright &y& Elsevier]

Abstract: This paper analyses the passive augmentation of convective heat transfer administering the nanoporous layers fabricated by electrochemical anodization and spray pyrolysis. Nanoporous structures fabricated in electrochemical anodization have pore size varying from 40 to 120 nm, and the pore size procured in spray pyrolysis fluctuates from 60 to 100 nm. Convective energy transfer greatly banks on surface attributes. These nanoporous structures aid in hindering the dynamic flow of fluid and the turbulence is achieved more expeditiously. The proliferation of the convective heat transfer obtained with electrochemically anodized nanoporous surface is 131% higher than the polished bare metals with surface roughness 0.2 μm. In case of spray pyrolysis the maximum proliferation is 120%. Disparate disciplines of nanoporous fabrication are perused for asserting a productive process. This paper also analyses the control parameters in the nanoporous fabrication process. [Copyright &y& Elsevier]

Abstract: Loop heat pipes (LHPs) are heat transfer devices whose operating principle is based on the evaporation/condensation of a working fluid, and which use the capillary pumping forces to ensure the fluid circulation. Their major advantages as compared to heat pipes are an ability to operate against gravity and a greater maximum heat transport capability. In this paper, a literature review is carried out in order to investigate how various parameters affect the LHP operational characteristics. This review is based on the most recent published experimental and theoretical studies. After a reminder of the LHP operating principle and thermodynamic cycle, their operating limits are described. The LHP thermal resistance and maximum heat transfer capability are affected by the choice of the working fluid, the fill charge ratio, the porous wick geometry and thermal properties, the sink and ambient temperature levels, the design of the evaporator and compensation chamber, the elevation and tilt, the presence of non-condensable gases, the pressure drops of the fluid along the loop. The overall objective for this paper is to point the state-of-the-art for the related technology for future design and applications, where the constraints related to the LHPs are detailed and discussed. [Copyright &y& Elsevier]

In this paper the optimization of fluid networks is based on the minimization of pumping power requirement. The total pipe network volume is constrained. It is shown that only in special cases the minimization of pumping power leads to the same architecture as the minimization of pressure drop or flow resistance. Fundamentals of fluid network optimization are developed for both spanning networks and networks where new non-consumer points are added (Gilbert–Steiner points). It is shown that networks with minimum pumping power must not contain loops. The influence of gravity on the optimization of flow configuration is also addressed. The principles developed in the paper are illustrated with an example representing a set of ten vertices to be connected with pipes. The paper provides designers with more effective basic tools for the conceptual design of fluid networks. [Copyright &y& Elsevier]

This paper presents a comprehensive review of the flow and heat transfer results of single-phase laminar mixed convection flow over vertical, horizontal and inclined backward- and forward-facing steps that have been reported in several studies in the open literature. The purpose of this paper is to give a detailed summary of the effect of several parameters such as step height, Reynolds number, Prandtl number, inclination angle, expansion ratio, temperature difference between the heated wall and the free stream, and buoyancy force (assisting and opposing) on the flow and thermal fields downstream of the step. Several correlation equations that were reported in many of these studies to predict the reattachment lengths of the recirculation regions that may develop upstream and/or downstream of the step are also summarized in this review. [Copyright &y& Elsevier]

This paper shows analytically and numerically how an originally uniform flow structure transforms itself into a nonuniform one when the objective is to minimize global flow losses. The flow connects one point (source, sink) to a number of points (sinks, sources) distributed uniformly over a two-dimensional domain. In the first part of the paper, the flow between neighboring points is modeled as fully developed through round tubes. It is shown that flow ‘maldistribution’ and the abandonment of symmetry are necessary for the development of flow structures with minimal resistance. The search for better flow structures can be accelerated: tubes that show a tendency of shrinking during the search can be assumed absent in future steps of structure optimization. In the second part of the paper, the flow medium is continuous and permeated by Darcy flow. The development of flow structures (channels) is modeled as a mechanism of erosion, where elements of the original medium are removed one by one, and are replaced with a more permeable medium. The elements selected for removal are identified based on two criteria: maximum pressure integrated over the element boundary, and maximum pressure gradient. The flow structures generated based on the pressure gradient criterion have consistently smaller flow resistances. As flow systems become smaller and more compact, the flow systems themselves become “designed porous media”. These design optimization trends revealed are generally applicable in constructal design, i.e., where miniaturization, global performance, compactness and complexity rule the design. [Copyright &y& Elsevier]

Three dimensional numerical simulations are carried out to explore the performance of vortex generators in a plate fin heat exchanger. Rectangular and perforated wings are used as vortex generators to enhance the heat transfer rates. A comparison is made between the performances of a plate fin with and without baffles. When the heat exchanger is equipped with baffles, the efficiency of two configurations was compared: a baffle with and without perforation. Also, the effects of the perforation shape were studied. It concerns three cases: rectangular, triangular and circular. Validation of our numerical results with the available experimental data has revealed a satisfactory agreement. The obtained results show that the baffled cases perform better than the unbaffled one. The performance factor is found to be higher in the perforated baffle than the baffle without perforation. Compared to the unbaffled case, the maximum thermal performance factor (TPF) of 2.14 was obtained with the circular perforated baffle, followed by the rectangular perforated baffle (TPF = 1.57), triangular baffle (TPF = 1.46) and finally the baffle without perforation (TPF = 1.41). At the end of paper, new correlations for the prediction of friction factor and Nusselt number depending on Reynolds number and the shape of perforation in baffles are developed. [ABSTRACT FROM AUTHOR]

The majority of this paper is to investigate effects of injection angles and velocity ratios (Vr) on thermal-hydraulic characteristics and thermal striping in a T-junction since the thermal striping is one of main degradation mechanisms for piping wall. Experimental measurement and observation are applied in the in-house T-junction. Considering the injection angle, a modified momentum ratio (M R *) is proposed to discriminate the flow pattern and to reveal the reverse flow phenomenon from the branch injection. The reverse flow upstream a T-junction can occur if M R * is less than approximately 0.05. The experimental results also reveal that strong thermal stratification exists near the intersection of a T-junction (x/D = 1) and uniform temperature distributions can be achieved as the mixing fluid passes through x/D = 4. The thermal striping in a T-junction can be investigated using the power spectrum density (PSD) versus frequency. Two distinct characteristics are revealed in these PSD spectra. The frequency for distinguishing these two regions is 9.0 Hz, which is similar to the value of 10 Hz in the previous work. In addition, PSD spectra are slightly higher for 90° injection than 45° injection and these spectra are insignificant to the Vr values. [ABSTRACT FROM AUTHOR]

The solid-liquid phase change process in a cavity with protruding heater is of significant importance to application of phase change material (PCM). In this paper, the phase change lattice Boltzmann (LB) model has been applied and the effects of heater and inclination angle of cavity on phase change process are investigated. The results show that moving the protruding heater is able to change the temperature distribution and location of solid-liquid interface. The phase change process can be accelerated by moving the heater to middle region of cavity. Decreasing the distance between heater and cavity upper wall enhances the flow obstruction, and reduces the heat transferred through upper and right walls of heater. Furthermore, rotating the cavity changes the distance of flow from heater to cavity. Increasing inclination angle from 30° to 45° enhances the heat transfer rate of heater's upper wall and weakens that of heater's right wall. [ABSTRACT FROM AUTHOR]

This paper presents the results of heat transfer of a multi-plate thermoacoustic system using complex Nusselt number in the presence of magnetic field. Assuming the applied magnetic field is perpendicular to the direction of the oscillating fluid flow, we derive the expressions for the fluctuating velocity and temperature from the governing unsteady-compressible-viscous forms of the continuity, momentum, and energy equations. These equations are simplified assuming small amplitude oscillations, a long wave, and a short stack. The hydrodynamic and thermal boundary layers are considered to be very small compared to the acoustic wavelength, and the longitudinal conduction heat transfer inside the boundary layers is assumed to be negligible. Both bulk mean and space averaged temperatures are considered as reference temperatures in the analytical solution. The complex Nusselt number equations are simplified and expressed as a function of the Hartmann number ( Ha δ ), the Swift number ( S w ), the modified Swift number ( S ¯ w ) and Rott’s functions ( f ν and f k ). The effect of Ha δ , S w , and S ¯ w on the Nusselt number is analyzed and presented graphically for both viscous and inviscid fluids. The value of Nusselt number in the boundary layer limit for the inviscid fluid is also analyzed. In the absence of a magnetic field, the simplified complex Nusselt number expression that is obtained by using the space averaged temperature as a reference temperature is compared with the data available in the literature and an excellent agreement is observed. This study will offer insight into ways to increase convection heat transfer rate, consequently help the thermoacoustic system designer to design a more power dense thermoacoustic system. [ABSTRACT FROM AUTHOR]

Controlling the dynamics in the weld pool is a highly demanding challenge in deep-penetration laser beam welding with modern high power laser systems in the multi kilowatt range. An approach to insert braking forces in the melt which is successfully used in large-scaled industrial applications like casting is the so-called Hartmann effect due to externally applied magnetic fields. Therefore, this study deals with its adaptation to a laser beam welding process of much smaller geometric and time scale. In this paper, the contactless mitigation of fluid dynamic processes in the melt by steady magnetic fields was investigated by numerical simulation for partial penetration welding of aluminium. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were solved based on temperature-dependent material properties up to evaporation temperature for two different penetration depths of the laser beam. The Marangoni convection in the surface region of the weld pool and the natural convection due to the gravitational forces were identified as main driving forces in the weld pool. Furthermore, the latent heat of solid–liquid phase transition was taken into account and the solidification was modelled by the Carman–Kozeny equation for porous medium morphology. The results show that a characteristic change of the flow pattern in the melt can be achieved by the applied steady magnetic fields depending on the ratio of magnetic induced and viscous drag. Consequently, the weld bead geometry was significantly influenced by the developing Lorentz forces. Welding experiments with a 16 kW disc laser with an applied magnetic flux density of around 500 mT support the numerical results by showing a dissipating effect on the weld pool dynamics. [ABSTRACT FROM AUTHOR]

This paper presents how the relationships between the Rayleigh–Taylor (RT) instability and the critical heat flux (CHF) and minimum heat flux (MHF) change under different experimental conditions. Experimental observations of the RT instability wavelength for various wire surface diameters and system pressures were analyzed to characterize the CHF and MHF in a pressurized wire pool boiling facility. Three types of heater diameter surfaces were considered: 0.3, 0.5, and 0.7 mm diameter bare Ni–Cr wires. The experimental system pressure ranged from 1 bar to 9 bar for observation of the RT instability wavelength changes in the CHF and MHF with the 0.5 mm diameter wire. High-speed video was used to analyze the changes in the RT instability wavelength under each experimental condition. The wavelengths at the CHF and MHF regions were characterized according to the heater diameter and system pressure. The results showed that using the change in the RT instability wavelength is a viable means of determining the variations in the CHF and MHF during pool boiling. [ABSTRACT FROM AUTHOR]

In this paper new results on linear viscoelastic thermal Marangoni convection are presented. The constitutive equation assumed is that of the Maxwell viscoelastic fluid. The competition between stationary and oscillatory convection is shown by means of plots of codimension-two points where the corresponding critical Marangoni numbers are the same. The variation of these points is investigated in a wide range of magnitudes of the thickness and thermal conductivity of the wall. Also, a discussion is given about the dependence they have on the Biot number of the fluid-atmosphere interface. Besides, it is shown how the range of the viscoelastic relaxation time corresponding to this points is modified by the Prandtl number. [ABSTRACT FROM AUTHOR]

An inverse method to determine transient fluid temperature with high accuracy is proposed. Thermometer housing is modelled as a solid cylinder. The temperature of the fluid is determined based on a transient temperature measured at the cylinder axis. To determine the temperature and heat flux on the surface of a cylinder a space marching method is used. Using the finite volume method, the heat balance equations are written for all control volumes into which the cylinder is divided. Then, the temperature in the control volume nodes is calculated sequentially, starting from the centre of the cylinder and then marching toward to its outer surface. The transient fluid temperature is calculated from the energy balance equation written for the node located at the outer surface of the cylinder. The heat transfer coefficient on the cylinder surface is determined from the experimental correlation based on the measured fluid mass flow rate or fluid velocity. Test calculations and experiments were carried out to validate the method. In the experimental study, the temperature of the superheated steam in the header was determined using three thermometers with different diameters: 7, 15 and 20 mm the mass flow rate of the steam flowing through the header was measured. Transient steam temperature was calculated using the method presented in the paper and compared with the actual steam temperature, which is known for measured pressure and enthalpy of the steam. [ABSTRACT FROM AUTHOR]

Due to the cyclical nature of the high pressure casting process, there will be a thermal periodic steady state of the high pressure die casting dies (hpdc-dies) during operation. A continuous temperature pattern (as a function of time), that is recurring during each process step can be observed. The quality of the castings as well as the life span of the tools is highly dependent on these periodic temperature patterns. It would be state of the art to simulate the hpdc-process with highly-specialized process simulation software. However, because of the high initial costs and the necessary human and computational resources, their use during operation to adjust the parameters of the cooling management is generally not utilized. Instead, the input parameters of the cooling management are chosen by trial and error. This paper focuses on the development a simple linear thermal model to describe the temperature patterns within the die. This is helpful to control the cooling management more effectively during operation. Approaches of dynamic systems simulation are used to create a model of the energy transport phenomena within the hpdc-die. Here it is shown in general terms how to model fluid-cooled solids by the combination of n sub process steps in n state space models to depict the whole thermal cyclical process. The set up of sets of balance equations are shown with a simplified example, using formal matrix operations. The obtained results of the simulation are discussed and evaluated. The conclusions of measured data from the experimental hpdc-die are compared to the solution of the simulation using the thermal model approach described above. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an experimental study of the phase change heat transfer inside a cylindrical latent heat energy storage system (LHESS), designed with a central finned copper pipe running the length of the cylindrical container, during charging and discharging operations. Longitudinal fins were added to the copper pipe to enhance the overall heat transfer rates during the phase change processes; fins with two orientations, straight fins and angled fins, are used. The phase change material (PCM) used is dodecanoic acid. The experimental work concentrates on studying the heat transfer mechanism during melting and solidification of the PCM, impacts of the heat transfer fluid (HTF) inlet temperature and HTF flow rates. Moreover, heat transfer enhancement effectiveness of straight fins and angles fins configurations is compared. It is observed that conduction is the dominant heat transfer mechanism during the initial stage of charging, and natural convection dominates once enough liquid PCM is present inside the system. Conduction dominates during the entire solidification process. Complete melting time is strongly affected by the HTF inlet temperature but very slightly by the HTF flow rates. [Copyright &y& Elsevier]

Abstract: This paper investigates the behaviour of two non-mixing fluids flowing in a heated curved channel. In simulating this immiscible fluid flow, a multi-phase model based on VOF approach is developed for steady, turbulent, incompressible fluid flow through curved channel. The analysis examines the influence of centrifugal forces arising from channel curvature on the phase distribution and flow patterns, and their impact on the thermal characteristics. Unique flow features are identified through the interactions between the fluid phases and vortex structures of the immiscible flow in curved channels. The convective heat transfer in curved channel is appraised in relation to the behaviour of fluid components and curved channel flow parameters. Finally, the study presents a Second-Law irreversibility analysis whereby a thermal optimisation technique is developed for immiscible fluid flow through heated curved channels. [Copyright &y& Elsevier]

Abstract: A novel passive cooling solution, Bio-mimic Perspiration Cooling (BP-Cooling), was recently proposed, which mimics the thermoregulation mechanism of living creatures to supply extra passive cooling on demand using an intelligent skin made from temperature sensitive hydrogel (TSHG). In this paper, the heat and mass transfer characteristics of BP-Cooling are investigated. The temperature and humidity fields of BP-Cooling are measured by the Twyman-Green interference technique and modeled by computational fluid dynamics (CFD) simulations. The validated CFD model is further used to study the impacts of different usage conditions, e.g. ambient temperature, ambient humidity, and the starting temperature of BP-Cooling, on the BP-Cooling performance. Results show that BP-Cooling can improve passive cooling performance up to twenty times above natural convection and may be powerful enough to enable next-generation mobile phones perform like personal computers in a wide design envelope. [Copyright &y& Elsevier]

Abstract: Nanofluids are a new class of heat transfer fluids that use dispersed nanometre particles in conventional base fluids, with the appropriate size and volume fraction capable of inducing an anomalous enhancement in the effective heat transfer coefficient of the mixture. The literature abounds with studies on several nanoparticles to be used on nanofluids engineering. Among those, carbon nanotubes are continuously referred to as the most promising nanoparticle as to thermal properties are concerned. Despite that, these nanoparticles apparently seem to be the ones gathering less attention from researchers since little experimental and theoretical studies can be found. The latter might be explained by the difficulty encountered in modelling some of the mechanisms observed in these particular systems. In this paper, an exhaustive critical analysis to the predictive models currently available for thermal conductivity of carbon nanotubes based nanofluids is presented. To this end, a statistical analysis of the different models available was carried out and it enabled to select specific nanofluid variables as control factors namely particle geometry, volume fraction, temperature and base fluid. The statistical study undertaken highlighted a lack of confidence on the models available since there is no convergence on the results. The latter seems to occur due to poor quality data, resulting in limited generic models. Therefore, this study revealed that more parametric experimental analysis must be given to specific control factors, namely carbon nanotubes length and volume fraction, since this seems to present higher contribution to the analytical results. [Copyright &y& Elsevier]

Abstract: In this paper, the convective heat transfer in viscoelastic creeping flow in a curved circular pipe is investigated analytically. The flow and heat transfer is assumed to be fully-developed and a constant heat flux at the walls is imposed as the boundary condition. The second order constitutive equation is used to simulate the low speed flow of viscoelastic fluids. The closed form of temperature distributions for H2 boundary condition is obtained using a perturbation method. For this reason, the pipe curvature ratio is used as the perturbation parameter for both flow field and temperature distribution. The solutions are obtained for two types of material models based on hypergeometric functions. Owing to the singularity situation, analyzing the effect of second normal stress difference on Dean flow is very complex. The current article therefore focuses on studying analytically the opposite effect of the first and negative second normal stress differences on convective heat transfer in viscoelastic Dean flow. The current study finds applications in polymer processing and arterial biofluid dynamics. [Copyright &y& Elsevier]

Abstract: This paper proposes characteristic time, length, velocity and heat flux scales for unsteady natural convection from an isothermal horizontal cylinder where the fluid properties yield a Prandtl number less than unity, a study that was not published in the current literature. These scales are compared with the results obtained from numerical simulations yielding acceptable accuracy. [Copyright &y& Elsevier]

Abstract: In this paper, we investigate theoretically the problem of steady laminar two-dimensional boundary layer flow and heat transfer of an incompressible viscous fluid in the presence of buoyancy force over an exponentially shrinking vertical sheet with suction. The shrinking velocity and wall temperature are assumed to have specific exponential function forms. The governing equations are first transformed to similarity equations using an appropriate similarity transformation. The resulting equations were then solved numerically using shooting technique involving fourth-order Runge–Kutta method and Newton–Raphson method. The influence of mixed convection/buoyancy parameter λ, suction parameter s and Prandtl number Pr on the flow and heat transfer characteristics is examined and discussed. Numerical results indicate that the presence of buoyancy force would contribute to the existence of triple solutions to the flow and heat transfer for particular value of pertinent parameters. It is different for the non-buoyant flow case i.e. when the buoyancy force is absent, the problem admits only dual solutions. Further, this study also reveals that the features of flow and heat transfer characteristics are significantly affected by buoyancy parameter λ, suction parameter s and Prandtl number Pr. [Copyright &y& Elsevier]

Abstract: The numerical model for description of flow dynamics and heat transfer in an impinging axisymmetric gas-droplets jet is presented. The Eulerian model uses for computations of the impinging gas-droplets jet. In this paper the two-phase turbulent jet is numerically predicted by the set of axisymmetic Reynolds averaged Navier–Stokes equations. The flow structure and heat transfer in the gas-droplets impinging spray with low mass fraction of droplets (M L1 ≤1%) is studied numerically. Gas phase turbulence is modeled with the use of Reynolds stress transport model for two-phase flow. Droplets addition causes a significant increase in heat transfer intensity (almost twice) in comparison with a single-phase impinging air jet in the stagnation zone. In the region of wall jet the heat transfer intensity in the two-phase impinging jet decreases and approaches the value of a single-phase impinging jet. [Copyright &y& Elsevier]

Abstract: As we know, the mean temperature difference approach perfectly solves the problem of thermal design of a two-fluid heat exchanger. However, so far, the equivalent method has not been found for that of three-fluid heat exchanger. In this paper, a mean temperature difference method called IMTD (integral-mean temperature difference) is developed in detail for a parallel stream three-fluid heat exchanger with two communications. It includes the derivation of IMTD formulae, the illustration of design and simulation procedures of IMTD thermal model, the corresponding exemplary calculations and discussion, and the validation of IMTD thermal model. The research shows that the derived IMTD formulae and accelerated method proposed in the present IMTD model significantly improve the design process in the past analytical models. It realizes a fast convergence which is usually within 10 steps for all parallel stream flow arrangements. Moreover, the present model has the same accuracy as past exact analytical models and simultaneously can be comparable to past approximate analytical models in simplicity. [Copyright &y& Elsevier]

Abstract: This paper addresses the influence of thermal non-equilibrium state on fully developed mixed convection in a vertical channel filled with porous medium. The non-Darcy-Brinkman-Forchheimer extended model has been considered. The governing equations are solved numerically by spectral collocation method and analytically for a special case (form drag equal to zero). The governing parameters for this problem are Rayleigh number (Ra), Darcy number (Da), Forchheimer number (F), inter-phase heat transfer coefficient (H), and porosity-scaled thermal conductivity ratio (γ). Special attention is given to understand the effect of thermal non-equilibrium parameters: H and γ on the rates of heat transfer and fluid flow profiles, for both buoyancy assisted as well as opposed cases. A comprehensive investigation indicates that, in case of buoyancy assisted flow, for each Ra considered in this study and when γ ≤ 1, there exists a minimum value Ho of H such that the heat transfer rate of fluid (Nuf ) at the wall is an increasing function in [0, Ho ]. Furthermore, as H is varied beyond Ho , Nuf decreases. Initially the decay of Nuf is fast, but later on it becomes smooth and slow. Similar results are also observed in buoyancy opposed case for very small values of γ. Variation of velocity profile as a function of γ indicates that change in velocity profile is sudden and abrupt. For H > 6, when γ = 0.01, Da = 10−2, |Ra| = 103 and F = 102, flow separation and point of inflection are completely died out from the profile. Overall, for both the cases, on increasing H makes the flow profile smooth (stabilizes the flow) and recovers the system equivalent to equilibrium state, whereas, γ destabilizes the flow by inviting point of inflection or flow separation on the velocity profile. [Copyright &y& Elsevier]

Abstract: The present paper presents a theoretical analysis of a cross flow heat exchanger with a new flow arrangement comprehending several tube rows. The thermal performance of the proposed flow arrangement is compared with the thermal performance of a typical counter cross flow arrangement that is used in chemical, refrigeration, automotive and air conditioning industries. The thermal performance comparison has been performed in terms of the following parameters: heat exchanger effectiveness and efficiency, dimensionless entropy generation, entransy dissipation number, and dimensionless local temperature differences. It is also shown that the uniformity of the temperature difference field leads to a higher thermal performance of the heat exchanger. In the present case this is accomplished thorough a different organization of the in-tube fluid circuits in the heat exchanger. The relation between the recently introduced “entransy dissipation number” and the conventional thermal effectiveness has been obtained in terms of the “number of transfer units”. A case study has been solved to quantitatively to obtain the temperature difference distribution over two rows units involving the proposed arrangement and the counter cross flow one. It has been shown that the proposed arrangement presents better thermal performance regardless the comparison parameter. [Copyright &y& Elsevier]

Abstract: This paper discusses an optimization problem of internal cooling passages within a turbine blade with Conjugate Heat Transfer (CHT) analysis involved. However, to make the problem computationally feasible it was necessary to reduce the CHT predictions by fixing the external flow and solving the task for the interior only (solid and coolant). The optimization is done with an evolutionary algorithm within a 30 dimensional design space which use of the Pareto approach. Results showed more reliable thermal field predictions comparing to the classical approach and possible improvements in the design obtained. [Copyright &y& Elsevier]

Abstract: The present paper focuses on problem of free convection fluid flow and heat transfer of Cu–water nanofluid inside L-shaped cavities. The governing equations are discretized using the finite volume method while the SIMPLER algorithm is employed to couple velocity and pressure fields. Using the developed code, the effects of the Rayleigh number, the aspect ratio of the L-shaped cavity, and the volume fraction of the Cu nanoparticles on the flow and thermal fields and heat transfer inside the cavity are investigated. The obtained results show that the average Nusselt number for all rage of cavity aspect ratio increases with increase in the Rayleigh number and the solid volume fraction of the nanofluid. Also the Rayleigh number in which transition from conduction to convection occurs postpones with decreasing the aspect ratio of cavity. Moreover it is found that the rate of heat transfer increases with decreasing the aspect ratio of the cavity. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental investigation of the hydrodynamic and thermal fields in a two-channel chevron-type plate heat exchanger for laminar and turbulent conditions. The friction factor for a Reynolds number up to 850 and the Nusselt number for the hot channel for a Reynolds number up to 1500 are presented. The qualitative influence of the Reynolds number on these two parameters is similar to that established in other published studies. The values of both these parameters obtained in the present study are intermediate between the corresponding extremes previously reported. The observed differences are attributed to the corrugation geometry, the presence of the straight smooth passages along the sides of the channels, the port configuration and the number of plates. On the other hand, the calculated effectiveness of the plate heat exchanger is identical with that obtained from the classical analytical expression for counter-flow heat exchangers. The temperature distributions on the first and the last of the three plates for a laminar case and two turbulent cases are also presented. Distorted isotherm patterns have been observed, likely caused by the presence of the straight smooth passages located along the longitudinal edges of the plates. [Copyright &y& Elsevier]

Abstract: This paper presents a comparison of experimental data and numerical predictions for the hydrodynamic and thermal fields in a two-channel plate heat exchanger. The temperature distributions on the first and the last of the three plates, the friction factor and the Nusselt number for each channel as well as the outlet temperatures of the two streams are compared. The laminar model has satisfactorily been validated for Reynolds numbers less than 400. For the turbulent flow regime, several two-equation models were tested and the Realizable κ–ɛ model with non-equilibrium wall functions has been found to give the closest results to the experimental data. The flow distribution in cross-sections of the channels was studied numerically. Streams of higher velocities on the channels’ sides, where two narrow vertical smooth passages are located, were observed. [Copyright &y& Elsevier]

Abstract: Micro heat sinks have a broad applicability in many fields such as aerospace applications, micro turbine cooling, micro reactors, electronics cooling, and micro biological applications. Among different types of micro heat sinks, those with micro pin-fins are becoming popular due to their enhanced heat removal performance. However, relevant experimental data in current literature is still scarce to adequately explain their differences from their macro size counter parts. In previous studies in literature, it was shown that thermal and hydrodynamic characteristics of micro pin-fin heat sinks are strongly affected by height over diameter (H/D) ratio of pin-fins. To address the lack of information about this subject, the objective of this work is to show how velocity boundary layer around pin-fins and consequently, the thermal and hydrodynamic characteristics are affected when H/D ratio and local Reynolds number (Re) vary. To investigate end wall effects, a small portion of a typical micro pin-fin heat sink is modeled. This portion is represented by a simplified model, which consists of a single pin-fin positioned in a rectangular micro channel. This approach simplified the micro heat sink, which is simulated for only half of it by using a symmetry plane. Moreover, the transverse channel walls are kept as close as the minimum distance (1.5D) between pin-fins available in the literature. In this paper, the pin-fin height over diameter ratio, H/D, varies from 0.5 to 5, while Reynolds number and heat flux provided from the fluid interacting surfaces of the micro pin-fin are in the range of 20≤ Re ≤150 and 100≤ q in (W/cm2)≤500, respectively. In this research, micro pin-fin heat sinks are three dimensionally modeled on a one-to-one scale with the use of commercially available software COMSOL Multiphysics 3.5a. Full and temperature dependent Navier–Stokes equations subjected to compressibility and energy equations are solved under steady state conditions. In order to validate the use of numerical models, simulation results are compared against theoretical predictions. The numerical results and theoretical predictions show a good agreement. After this validation, parametric analysis is performed using the three dimensional model developed with COMSOL Multiphysics 3.5a. The end wall effects are quantified, and this amount decreases with Re and H/D. It is revealed that end walls play an important role on the total fluidic force acting on the micro pin-fin and on the heat transfer coefficients. Moreover, the trends in the amount of end walls effects, the ratio of viscous over total forces on the pin-fin, friction factors, and Nusselt numbers change at various critical Reynolds numbers. It is also demonstrated that increasing H/D ratio leads to a less stable flow, higher fluidic forces on the micro pin-fin with an increased partial role of viscous forces relative to pressure forces, smaller friction factors, and higher heat transfer coefficients. There are maxima and minima in Nusselt number profiles for different H/D ratios. It is found that increasing Re has a positive role in Nusselt numbers, as well as a parallel effect with H/D on fluidic forces on micro pin-fin, friction factors, and heat transfer coefficients. Different than the effect of H/D, Re decreases the partial role of viscous forces relative to pressure forces. [Copyright &y& Elsevier]

Abstract: In this paper we solve, numerically and asymptotically, the steady-state analysis of a conjugate heat transfer process in an electro-osmotic and fully developed laminar flow including Joule heating effects. In addition, the viscoelastic fluid obeys the simplified Phan-Thien-Tanner (SPTT) constitutive equation. Taking into account the finite thermal conductivity of the micro-channel wall, the dimensionless temperature profiles in the fluid and solid wall have been obtained as functions of the dimensionless parameters involved in the analysis: a conjugate parameter, α, which represents the competition between the longitudinal conductive heat in the micro-channel wall to the convective heat transfer in the fluid; , a parameter that describes the viscoelastic behavior of the fluid; the well-known Peclet number, Pe; a normalized power generation term, Λ, being the ratio of heat flux from the external wall to the Joule heating (and smaller or equal to unity); the ratio of pressure to the electro-osmotic forces, Γ; and the aspect ratios of the micro-channel and the solid wall, β and ɛ, respectively. The results for the temperature fields, in the fluid and micro-channel wall show a strong dependence of the above dimensionless parameters, therefore, this set of parameters controls directly the thermal performance of this micro-channel model. [Copyright &y& Elsevier]

Abstract: A computational model is developed to describe the thermo-fluid-dynamic behaviour of a helically coiled steam generator device working with water and widely adopted in the nuclear industry. The discretized governing equations are coupled using an implicit step by step method. The mathematical model includes: a subcooled liquid region, a two-phase flow region, and a superheated vapour region (according to the once-through nature of the heat exchanger). All the flow variables (enthalpies, temperatures, pressures, vapour qualities, velocities, heat fluxes, etc.), together with the thermo-physical properties, are evaluated at each point of the grid in which the domain is discretized. A full-scale experimental investigation carried out at SIET thermal-hydraulics labs in Piacenza (Italy), and aimed at characterizing the fluid-dynamic behaviour of two-phase flows in helically coiled tubes, is referenced in the present paper. Two-phase pressure drops data reduction allowed optimizing a suitable form of the friction factor multiplier required by momentum balance equation. Comparisons of the numerical simulations with a wide range of two-phase pressure drops measurements (experiments conducted both in diabatic and adiabatic conditions) are shown in order to validate the proposed model. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the effects of volumetric heat sources on natural convection heat transfer and flow structures in a wavy-walled enclosure are studied numerically. The governing differential equations are solved by an accurate finite-volume method. The vertical walls of enclosure are assumed to be heated differentially whereas the two wavy walls (top and bottom) are kept adiabatic. The effective governing parameters for this problem are the internal and external Rayleigh numbers and the amplitude of wavy walls. It is found that both the function of wavy wall and the ratio of internal Rayleigh number (Ra I) to external Rayleigh number (Ra E) affect the heat transfer and fluid flow significantly. The heat transfer is predicted to be a decreasing function of waviness of the top and bottom walls in case of and . [ABSTRACT FROM AUTHOR]

Abstract: This article analyzes parallel flow microchannel heat exchangers with non-adiabatic thermal condition imposed on the wall separating the fluids and the ambient. For purposes of analysis, a thermal model comprising of two governing equations, one for each of the fluids, is developed and analytically solved to obtain equations for calculating the axial temperature and thereby the effectiveness of both fluids. Provision is provided in these equations for the temperature of the ambient interacting with the hot fluid to be different from that associated with the cold fluid. Analytical equations for determining the heat transfer between the individual fluids and the respective ambient as well as that between the fluids are also developed in this paper. The temperatures/effectiveness of the fluids depend on parameters such as NTU, fluid heat capacities, thermal resistance between the individual fluid and the respective ambient and ambient temperatures. In addition, under certain operating conditions the phenomenon of temperature-cross is observed. Depending on the ambient temperatures, increase in NTU will continuously improve the effectiveness of one the fluids; the effectiveness of the other fluid over the same NTU range increases before peaking and subsequent decrease. Equations for determining the NTU at which this peak value in effectiveness occurs are also provided. An unbalanced flow microchannel heat exchanger with the hot fluid having the lowest heat capacity has better effectiveness than when the cold fluid has the lowest heat capacity. [ABSTRACT FROM AUTHOR]

Abstract: Fluid flow with phase change heat transfer in a three-dimensional porous channel with asymmetrically heating from one side is numerically studied in this paper. The “modified” Kirchhoff method is used to deal with the spatial discontinuity in the thermal diffusion coefficient in the energy equation. The velocity and temperature fields, as well as the liquid saturation field on the heated section of the wall with different Peclet and Rayleigh numbers are investigated. The results show that the liquid flow bypasses the two-phase zone, while the vapor flows primarily to the interface between the sub-cooled liquid zone and the two-phase zone. An increase in the Peclet number decreases the two-phase region while an increase in the Rayleigh number helps to spread the heat to a larger region of the domain. The distribution of the liquid saturation on the heated section of the wall indicates that the minimum liquid saturation increases with the increase of both the Peclet and Rayleigh numbers. [ABSTRACT FROM AUTHOR]

Abstract: An experimental investigation of the performance characteristics of a counter flow wet cooling tower represented by the heat rejected by the tower and its thermal effectiveness is presented in this paper. The tower is filled with a “VGA.” (Vertical Grid Apparatus) type packing which is 0.42 m high and contains four (04) galvanised sheets having a zigzag form, between which are disposed three (03) metallic vertical grids in parallel with a cross-sectional test area of 0.15 m × 0.148 m. The investigation is concerned mainly on the effect of the air, water flow rates and the inlet water temperatures on the thermal effectiveness of the cooling tower as well as the heat rejected by this tower from water to be cooled to the air stream discharged into the atmosphere. The two operating regimes which were observed during the air/water contact inside the tower, a Pellicular Regime (PR) and a Bubble and Dispersion Regime (BDR) appear to be important, as The BDR regime enables to cool larger amount of water flow rates, while the Pellicular regime results with higher thermal effectiveness. [ABSTRACT FROM AUTHOR]

Abstract: Double dispersion in an open end cavities are simulated using Lattice Boltzmann Method (LBM). The flow is driven by the buoyancy effect due to the heated vertical wall and species concentration at the heated wall of the cavity (closed end). The paper is intended to address the physics of flow, heat and mass transfers in open ended cavities and close end slots. Prandtl number (Pr) is fixed to 0.71 (air) for the thermal Rayleigh number (RaT ) of 104, 105 and 106. The results are presented for moderate Lewis number of 2, 4 and 8 and for a range of buoyancy ratio, N, (species to thermal). The species concentration induced buoyancy force either aids or opposes the thermally driven flow, which is determined by the value of buoyancy ratio (positive or negative, respectively). Interesting flow patterns were predicted for opposing buoyancy forces. [ABSTRACT FROM AUTHOR]

Abstract: In the present paper, a numerical model of spot pulsed current GTA welding for partially and fully penetrated weld pools is presented. Heat transfer and fluid flow in the weld pool driven by the combination of electromagnetic force, buoyancy force, surface tension gradient and latent heat are included in our model. A new formulation of the electromagnetic problem is introduced to take into account eddy current in the weld pool. The shape of the free deformable surface under the action of pulsed arc force is also handled after the magneto-hydrodynamic calculation. The numerical model was applied to 304 stainless steel welding. We compare the influence of various pulsed welding parameters such as pulse frequency and current ratio on the weld quality. Experimental study is conducted to compare our numerical prediction with welding macrographies. It shows a good agreement of the model. [Copyright &y& Elsevier]

Abstract: This work is focussed on the development of a numerical simulation model that predicts the thermal and fluid-dynamic behaviour of the two-phase flow distribution in systems with multiple branching tubes like manifolds. The geometry of a simulated branching system is represented as a set of tubes connected together by means of junctions. On one side, the in-tube evaporation/condensation phenomena are simulated by means of a one-dimensional two-phase flow model, and on the other side, the splitting/converging flow phenomena occurring at junctions are predicted with appropriate junction models obtained from the technical literature. The global flow distribution is calculated using a semi-implicit pressure based method (SIMPLE-like algorithm) where the continuity and momentum equations of the whole domain are solved and linked with both the in-tube two-phase flow model and the junction models. In the present paper, the flow distribution model is described and its most significant aspects are detailed. Furthermore, the model is validated against experimental and numerical data found in the open literature. The numerical predictions are compared against an adiabatic single-phase flow manifold system working with water and also against a two-phase flow upwardly oriented manifold system working with carbon dioxide. In addition to this, a numerical comparison of a manifold system with two different orientations is carried out. Concluding remarks about the possibilities that this kind of model offers are presented in the last section. [Copyright &y& Elsevier]

Abstract: This paper studies the heat transfer process in a two-dimensional steady hydromagnetic natural convective flow of a micropolar fluid over an inclined permeable plate subjected to a constant heat flux condition. The analysis accounts for both temperature dependent viscosity and temperature dependent thermal conductivity. The local similarity equations are derived and solved numerically using the Nachtsheim–Swigert iteration procedure. Results for the dimensionless velocity and temperature profiles and the local rate of heat transfer are displayed graphically delineating the effect of various parameters characterizing the flow. The results show that in modeling the thermal boundary layer flow when both the viscosity and thermal conductivity are temperature dependent, the Prandtl number must be treated as a variable to obtain realistic results. As the thermal conductivity parameter increases, it promotes higher velocities and higher temperatures in the respective boundary layers. The wall shear stress increases with the increase of thermal conductivity parameter. This is true of electrically conducting as well as electrically non-conducting fluids. The presence of heat generation invigorates the flow and produces larger values of the local Nusselt number compared with the case of zero heat generation. [Copyright &y& Elsevier]