Thermal management is essential to compact devices particularly for high heat flux removal applications. As a popular thermal technology, refrigeration cooling is able to provide relatively high heat flux removal capability and uniform device surface temperature. In a refrigeration cycle, the performance of evaporator is extremely important to the overall cooling efficiency. In a well-designed evaporator, effective flow boiling heat transfer can be achieved whereas the critical heat flux (CHF) or dryout condition must be avoided. Otherwise the device surface temperature would rise significantly and cause device burnout due to the poor heat transfer performance of film boiling. In order to evaluate the influence of varying imposed heat fluxes, saturated flow boiling in the evaporator is systematically studied. The complete refrigerant flow boiling hysteresis between the imposed heat flux and the exit wall superheat is characterized. Upon the occurrence of CHF at the evaporator wall exit, the wall heat flux redistributes due to the axial wall heat conduction, which drives the dryout point to propagate upstream in the evaporator. As a result, a significant amount of thermal energy is stored in the evaporator wall. While the heat flux starts decreasing, the dryout point moves downstream and closer to the exit. The stored heat in the wall dissipates slowly and leads to the delay in rewetting or quenching, which is the key to understand and predict the flow boiling hysteresis. In order to reveal the transient heat releasing mechanism, an augmented separated-flow model is developed to predict the moving rewetting point and minimum heat flux at the evaporator exit, and the model predictions are further validated by experimental data from a refrigeration cooling testbed.

Microscale heat transfer in macro geometry has been proven to yield comparable heat transfer performance to that of typical microchannels. This paper takes it a step further by looking at three passive heat transfer enhancement techniques to improve the heat transfer performance of the newly proposed system for single-phase liquid flow. The novelty of the study lies in that the enhancement features are designed based on inspiration from nature. Fish scale, Durian (a thorny tropical fruit), and Inverted Fish Scale enhancement profiles are considered. In this study, an annular microchannel is formed by securing a cylindrical insert of mean diameter 19.4 mm within a cylindrical pipe of internal diameter 20 mm. The enhancement features are introduced on the surface profile of the insert, while the heat is supplied to the flow via the cylindrical pipe of fixed surface area. Therefore, heat transfer is improved by increasing convective heat transfer coefficient, for a constant heat transfer area. The enhancement features serve to increase heat transfer coefficient by disturbing the flow and thermal boundary layers. Experiments were carried out to investigate the effect of the three enhancement profiles on the heat transfer and flow characteristics of the microscale flow. The extent of enhancement is computed with the Plain profile as benchmark. The constant parameters include microchannel length of 30 mm, mean hydraulic diameter of 600 μm, and heat input of 1000 W. Reynolds number range is 1,300 to 4,600, with water as working fluid. Results show that the Inverted Fish Scale profile doubles the Nusselt number as compared to the Plain profile. However, when friction factor increment is considered, Durian profile yields the best overall thermal performance, nearly 1.4 times better than the Plain profile. In the whole study, the maximum convective heat transfer coefficient achieved is 45.0 kW/m 2 ·K, using Inverted Fish Scale profile at Reynolds number of 4,300. The pressure drop values of the system are all less than 3 bars, which may easily be achieved by a commercially available pump.

The heat dissipation of current busbur in power plant is one of the important issues in power transmission, usually through the cylinder slotted to strengthen heat dissipation. Natural convection in a cylinder with an internal slotted annulus is the computational model abstracted from it. Natural convection in a cylinder with an concentric slotted annulus is concerned. Attention is focused on the effects of different slotted sizes on natural convection. Numerical results showed that, the equivalent thermal conductivity increases with the increase of Rayleigh number. At high Ra, the system heat transfer exhibit rich nonlinear characteristics. When the slotted direction or the slotted degree changed, it would have an important impact on the flow and heat transfer in the system, and also influence the related nonlinear characteristics.

The successful exfoliation of atomically-thin bismuth telluride quintuple layer (QL) attracts tremendous interest in investigating the electron and phonon transport properties in this quasi-two-dimensional material. While experimental results show that thermal conductivity is significantly reduced in Bi 2 Te 3 QL compared to the bulk phase, the underlying mechanisms for the reduction is still unclear. Also in some measurements, the Bi 2 Te 3 QL is usually supported on the substrate and the effect of the substrate on heat transfer in Bi 2 Te 3 QL is unknown. In this work, we have performed molecular dynamics simulations and normal mode analysis to study the mode-wise phonon properties in freestanding and supported Bi 2 Te 3 QL. We found that the existing of substrate will decrease the phonon relaxation times in Bi 2 Te 3 QL in the full frequency range. Thermal conductivity accumulation function for both freestanding and supported Bi 2 Te 3 QL are constructed and compared. We found that half of heat transfer in freestanding Bi 2 Te 3 QL contributed from phonons with mean free paths larger than 16.5 nm, while in supported Bi 2 Te 3 QL this value is reduced to 11 nm. In both cases phonons with MFPs in the range of 10–30 nm are the dominate heat carriers, which contribute to 55% and 53% of thermal conductivity in freestanding and supported cases.

In this study, the effect of pHEMA (Polyhydroxyethylmethacrylate) nanostructure coated surfaces on flow boiling was investigated in a rectangular microchannel. Experiments were conducted using deionized water as the working fluid to investigate flow boiling in a microchannel with dimensions of 14 cm length, 1.5 cm width, and 500 μm depth. The effect of pHEMA coatings (coated on 1.5 × 1.5 cm 2 silicon plates) on heat transfer coefficients and flow patterns was assessed and supported using a high speed camera system. Although the contact angle decreases on nano-coated surfaces, due to surface porosity, boiling heat transfer coefficient increases. Furthermore, visualization results indicated that uncoated surfaces experienced a smaller nucleate boiling region. It was also observed that dryout occurs at higher heat fluxes for coated surfaces.

With the development of metamaterials, microscale thermal cloak attracted many researchers’ attention. It was found that a thermodynamic cloak has unique characteristics of heat transfer transformation, with fundamental principle of transformation optics applied in thermodynamic field. An overview of thermal cloak related studies have not explained the physical mechanism in view of energy aspect. In the current work, two-dimensional heat transfer model of a multilayer thermal cloak was investigated through simulation according to Schittny’s microstructured model as well as an only polydimethylsiloxane protected layer model and plate model as control groups. Numerical simulations were developed with ANSYS FLUENT software for three models respectively in the process of heating to analysis the heat transfer and stealth protection during transient heat transfer process. The simulation results were agreed well with other’s previous experiment results on temperature distribution. The cloaking mechanism was analyzed by entropy generation approaches, and deriving the thermodynamics explanation at the aspect of the energy transfer. Thermodynamic cloak structure has a good heat stealth effect on the process of heat transfer, without the effecting outside the protected object on the distribution of both temperature and energy. Thermodynamic cloak control the energy dissipation inside the multi-layer structure, but the maximum dissipation position was shifted along the heat transfer processing.

As the rapid growing of the semiconductor logic gate number and operation speed, the heat dissipated from electronic devices increases drastically. Moreover, most of the heat flux can reach about 100 W/cm 2 , therefore efficient removal of the heat from the electronic devices is essential to ensure the reliable operation of the electronic devices. The traditional direct cooling system, such as air cooling, liquid cooling, would not be able to transfer the high heat flux owing to their heat transfer limits, so advanced cooling solutions are necessary. The flat heat pipes have some advantages, such as small scale, strong heat transfer capacity, low weight penalty and low environmental requirements, therefore, in recent years, researchers have shown great interest for the flat heat pipe. But most of them played the important on the structure design of the flat heat pipes, and few of them focused on the study of the effect of the working fluid on the heat transfer performance. In this paper, a flat heat pipe with rectangular channel is designed and manufactured, and an experimental set up was built to study working fluid on the effects of the flat heat Pipe thermal performance. The flat heat pipe is heated via a 35mmx20mm rectangular electrical resistance (the evaporator side), and the other side (the condenser side) is cooled by convection of a heat sink. In the experimental work, three types of working fluid are used in the heat pipe: (A) deionized water, (B) deionized water-based Fe3O4 nano fluid (1, 1.5wt%). A comparison is performed for the thermal performance of different size flat heat pipe. Finally, the experimental results showed that nano fluid could improve the thermal performance of the FHP. With the same charge volume, the heat transfer coefficient of the FHPs filled with nano fluid were higher than that of DI water. There was an optimal mass concentration which was estimated to be 1.5 wt% to achieve the maximum heat transfer enhancement.

The heat transport capability in an oscillating heat pipe (OHP) significantly depends on the oscillating frequency. An external frequency directly affects the natural frequency in the system. In this investigation, the ultrasound sound effect on the heat transport capability in an OHP was conducted with focus on the ultrasonic frequency effect on the oscillating motion and heat transfer capacity in an OHP. The ultrasonic sound was applied to the evaporating section of the OHP by using the electrically-controlled piezoelectric ceramics. The heat pipe was tested with or without the ultrasonic sound with different frequencies. In addition, the effects of operating temperature, heat load from 25 W to 150 W were investigated. The experimental results demonstrate that the heat transfer capacity enhancement of the OHP depends on the frequency of the ultrasound field, and there exists an optimum combination of the frequencies which will lead to the largest enhancement of the heat transfer capacity of the OHP.

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently. Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. A range of cross-sectional geometries for the curved channels were compared, showing significantly higher Nusselt Numbers than equivalent straight channels throughout, and finding superior performance factors for square, circular and symmetrical trapezoidal profiles. Due the difficulty and expense in manufacturing circular microchannels, the relatively simple to fabricate square and symmetrical trapezoidal channels are put forward as the most advantageous designs. These results take into account both constant wall temperature and constant heat flux conditions. For a given set of channel dimensions, an optimal input flow rate condition is also determined.

The dimension of electronic devices becomes smaller and smaller and, thus, it is of crucial importance to enhance the heat dissipation from such tiny devices. The present study investigates the boiling heat transfer in a minichannel heat sink with saw-tooth structure on channel surface. The heat sink is comprised of four minichannels with hydraulic diameter of 0.8 mm and made of copper. The dimensions of the base area of the heat sink are 90 mm (length) × 5 mm (width). The saw-tooth topology on the bottom surface of minichannel was manufactured by wire-cut electrical discharge machining (EDM). The height, tip angle, and pitch of the saw-tooth structure are 0.5 mm, 45°, and 1mm, respectively. This study employed refrigerant HFE-7100, which is of low global warming potential (GWP), as a working fluid to investigate the boiling heat transfer in three kinds of surface structures (i.e., plain, parallel saw-tooth, and counter saw-tooth). The mass flux of the HFE-7100 ranged from 64 to 285 kg/m 2 s. The experimental results showed that the critical heat flux (CHF), compared to the plain minichannel, is improved by 46.7% and 40.2%, respectively, in the parallel and counter saw-tooth minichannels for a low mass flux of 127 kg/m 2 s. This result indicated that the CHF is considerably enhanced by the saw-tooth structure with both parallel and counter flow designs for the low mass flux. However, the CHF in the parallel and plain minichannels is nearly the same for a large mass flux of 285 kg/m 2 s. But for a saw-tooth structure with counter flow design, the CHF increases by 17.1% compared to the plain minichannel. Consequently, the experimental results demonstrated that the CHF can be enhanced by using saw-tooth structure on the channel surface.

The Present investigation has been carried out to study the performance of nano enhanced phase change material (NEPCM) based heat sink for thermal management of electronic components. Enthalpy based finite volume method is used for the analysis of phase change process in NEPCM. To enhance the thermal conductivity of phase change material (PCM), copper oxide nano particles of volume fractions 1%, 2.5% and 5% are added to PCM. A heat flux of 2500 W/m 2 is taken as input to the heat sink. The thermal performance of the heat sink with PCM is compared with NEPCM for each volume fraction of nano particle for both finned and unfinned configurations. It is observed that the nano particle volume concentration plays a major role in removing the heat from the chip in case of unfinned heat sink configuration. However, for finned heat sink configuration, the volume concentration effect is not appreciable. In addition, the performance of NEPCM based finned heat sink is studied under cyclic loading in both natural and forced convection boundary conditions. It is observed that under forced convection the solidification time is reduced.

For the state of condensation in tube, liquid condensate separation in middle process can prolong the state of steam entrance region of higher heat transfer coefficient. It is called short-tube effect theory. Combined with the traditional condenser, a shell and tube condenser was designed for experiment research in this paper, and compared with the traditional condenser by opening liquid distribution pipes arranged in both sides of condenser. The results showed that liquid distribution pipes with different diameter have different condensation effect. Under the same steam flow rate of inlet, liquid distribution pipes with different combination of diameter and number indicated that its coefficient of heat transfer are higher than the traditional heat transfer by 14.2%, 15.5% and 25.1%. This result illustrated that heat exchange efficiency of a shell and tube condenser with liquid distribution pipes is better than a traditional condenser.

In this paper, the cross-plane thermal conductance σ of multi-layer graphene nanobundles (MLGNBs) is investigated using the non-equilibrium Green’s function method. For the normal MLGNBs, the σ has a positive dependence on the lateral area S due to more atoms involved in the heat transport in the larger S. However, the thermal conductance per unit area Λ is negative dependent on the S since high-frequency phonons contribute less to Λ with low transmission function and small number while the increased phonon branches are mainly located in the high-frequency range. Interestingly, as the S is larger than several square nanometers, the Λ converges to the macroscopic value, independently on the S. Then the staggered MLGNBs is investigated, the results show that increasing both staggering distance between neighboring graphene layers with each other and the graphene layer number in the central device can modulate the σ in a large scope due to the boundary scattering. Finally, in the MLGNBs junction, we found the variation of heat flux direction has an important effect on the σ while the layer number in the central device has weak effect on the cross-plane thermal transport. Our results help understand the cross-plane thermal transport of MLGNBs and provide a model to investigate the thermal property of layered material nanobundles.

Multiphase flow phenomena in single micro- and minichannels have been widely studied. Microchannel heat exchangers offer the potential for high heat transfer coefficients; however, implementation challenges must be addressed to realize this potential. Maldistribution of phases among the microchannels in the array and the changing phase velocities associated phase change present design challenges. Flow maldistribution and oscillatory instabilities can severely affect heat and mass transfer rates as well as pressure drops. In components such as condensers, evaporators, absorbers and desorbers, changing phase velocities can change prevailing flow regimes from favorable to unfavorable. Geometries with serpentine passages containing pin fins can be configured to maintain favorable flow regimes throughout the length of the component for diabatic phase-change heat and mass transfer applications. Due to the possibility of continuous redistribution of the flow across the pin fins along the flow direction, maldistribution can also be reduced. These features enable the potential of high heat transfer coefficients in microscale passages to be fully realized, thereby reducing the required transfer area, and achieving considerable compactness. The characteristics of two-phase flow through a serpentine passage with micro-pin fin arrays with diameters 350 μm and height 406 μm are investigated here. An air-water mixture is used to represent two-phase flow through the serpentine test section, and a variety of flow features are visually investigated using high-speed photography. Improved flow distribution is observed in the serpentine geometry. Distinct flow regimes, different from those observed in microchannels are also established. These observations are used to obtain void fraction and interfacial area along the length of the serpentine passages and compared with the corresponding values for straight microchannels. Models for the two-phase frictional pressure drops across this geometry are also developed.

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently. Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. These studies examined the variation of Nusselt Number over the length of the channel, for a range of different curvatures (and hence Dean numbers). The results showed significantly higher heat transfer occurring in curved channels, especially in areas where the generated Dean vortices are strongest, with the variation in Nusselt Number forming the shape of an ‘arc’. In this way, a relationship between the Dean Number and the Nusselt Number is characterised and discussed, leading to suggestions regarding optimal microfluidic heat transfer design.

In this work, the interfacial thermal conductance across Cu/graphene/Cu interfaces is investigated using the density functional theory (DFT) and the nonequilibrium Green’s function (NEGF) method. In order to study how hydrogenation of graphene affects thermal transport behaviors at the interfaces of Cu/graphene/Cu, we also analyze the interfacial thermal conductance across Cu/hydrogenated-graphene/Cu (Cu/H-graphene/Cu) with both double-sided and single-sided hydrogenated graphene. Our results show that, the interfacial thermal conductance across Cu/H-graphene/Cu interfaces is almost twice of the value across Cu/graphene/Cu interfaces. For Cu/H-graphene/Cu with double-sided hydrogenated graphene (Cu/DH-graphene/Cu), the hydrogen atoms between graphene and Cu layers provide additional thermal transport channels. While for Cu/H-graphene/Cu with single-sided hydrogenated graphene (Cu/SH-graphene/Cu), the hydrogen atoms not only provide additional thermal transport channels at the hydrogenated side of graphene, but also reduce the equilibrium separation between graphene and Cu layers at the non-hydrogenated side of graphene due to the transfer of massive electrons, which enhances the interface coupling between graphene and Cu layers. The phonon transmission shows that both double-sided and single-sided hydrogenation of graphene can increase the heat transport across the interface. Our calculation indicates that the interfacial thermal conductance of Cu/graphene/Cu nanocomposition can be improved by hydrogenation.

An incompressible electrically conducting viscous fluid flow influenced by a local external magnetic field may develop vortical structures and eventually instabilities similar to those observed in flows around bluff bodies (such as circular cylinder), denominated magnetic obstacle. The present investigation analyses numerically the three-dimensional flow and heat transfer around row of magnetic obstacles. The vortex structures of magnetic obstacles, heat transfer behaviors in the wake of magnetic obstacles and flow resistance are analyzed at different Reynolds numbers. It shows that the flow behind magnetic obstacles contains four different regimes: (1) one pair of magnetic vortices, (2) three pairs namely, magnetic, connecting, and attached vortices, (3) smaller vortex shedding from the in-between magnetic obstacles, i.e. quasi-static and (4) regular vortex shedding from the row of magnetic obstacles. Furthermore, downstream cross-stream mixing induced by the unstable wakes can enhance wall-heat transfer, and the maximum value of percentage heat transfer increment (HI) is equal to about 35%. In this case, the thermal performance factor is more than one.

An analysis for the cross-flow heat exchanger is conducted for electronic cooling applications, with the design goal of dissipating 175W from high power chip by maintaining the chip temperature within 85 °C in a compact space. Liquid to liquid heat exchanger in cross flow arrangement is preferred due to its compact size and high effectiveness. A volume averaging formulation is developed to determine the heat transfer coefficient at the unit cell level. The effects of channel shape, channel size, and heat exchanger material are examined through the heat transfer in the unit cell model. The obtained heat transfer coefficients are also used for the estimation of the heat exchanger thermal performance based on the effectiveness-NTU method. To verify the volume averaging formulation, a full field heat and fluid flow over the cross-flow heat exchangers are investigated through numerical computation. The amount of heat exchanged is extracted and compared with the unit cell model prediction. A fairly good agreement is obtained between the two approaches. Fabrication of cross-flow heat exchanger is further discussed to meet the design target.

Graphene nanoplatelets (GNPs) are a kind of material with excellent thermal conductivity. It can significantly improve the heat-conducting property of epoxy resin (EP) matrix. In this study, GNPs/EP composites were prepared by ultrasonication and the cast molding method. The effect of optimization in the preparation process on the thermal transfer performance of the composites was discussed. The pulverizing time of GNPs and three-roll grinder grinding of composites were considered. The results indicated that when the mass fraction of GNPs was 4.3%, in its pulverizing time of 2s, the thermal conductivity of GNPs/EP composites was up to 0.99 W/m·K, and it was increased by 9% compared with non-pulverization treatment. However, after pulverizing two seconds and grinding three times, the thermal conductivity of the composite reached the maximum (1.06 W/m·K) when the mass fraction of GNPs was 4.3%, and it was finally increased by 307.7% compared with epoxy resin matrix.

A simultaneous visualization and heat performance of oscillating heat pipes (OHPs) were performed. Experiments were performed under different surface wetting characteristics. Results showed that the start-up performance was improved on hydrophilic OHP as opposed to the copper OHP. A small bubble grew quickly and became a vapor plug in the evaporation section with hydrophilic surface. The process of vapor expansion and contraction accompanying liquid slug movement upward and backward continued to occur as a spring, and the OHPs started up. However, the hydrophobic OHP failed to start up. For the superhydrophobic OHP, nucleate boiling took place in the evaporation section, and the bubble expansion and contraction phenomenon were not observed. Heat transfer results showed that wall temperature fluctuations were observed at the start-up stage. The start-up time for the hydrophilic OHP was lowest and the amplitudes of temperature oscillations were increased in hydrophilic OHP compared to the copper OHP.