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Pulse firing is one of the major operation modes of bipropellant thrusters for the attitude control of small-scale spacecrafts. In the pulse-firing mode, the operational range of the thruster depends on the deterioration of liquid film cooling performance. Because cooling performance is determined by the balance of heat removed by the liquid film and transferred to the manifold, the behavior and heat transfer characteristics of liquid films under the intermittent injection of liquid jets must be understood to enable a wider range of operational patterns. We conducted cooling tests on a superheated metal plate via the pulsed injection of a liquid jet for better understanding of the pulsed cooling process. Different injection patterns, including continuous injection, with the same injection quantity were examined on two types of metal plates (aluminum alloy and copper), because the thermal properties of metal plates affect both the heat transfer characteristics of the liquid film and temperature rise of the plate during the inter-pulse duration. The cooling process was evaluated based on the evolution of the liquid film on the metal plate and rear-side temperature of the metal plate. The evolution and temperature were simultaneously visualized using a high-speed camera and infrared camera, respectively. To link the liquid film state to the heat transfer process, the temperature and heat flux on the cooled surface were estimated by solving the inverse problem of three-dimensional transient heat conduction. The results indicate that the wetting front position corresponds to the position of the maximum temperature gradient. Additionally, the residual liquid film is consumed through the evaporation and the droplet dispersion related to the nucleate boiling. The effects of thermal inertia and diffusivity of the metal plate highlight the differences in the amount of heat removed by the liquid film. The heat removed by the liquid film during pulsed cooling exhibited a peak and was higher than that removed by continuous injection on the aluminum alloy plate. Less heat was removed under low-duty-cycle conditions, and the amount of heat removed by the liquid film was lower than that removed by continuous injection on the copper plate.

Copyright © 2022 The Author(s). Muti-channel flat heat pipe is an innovative technology recently used at the rear of photovoltaic cells to absorb and reuse the wasted heat. To better understand the fundamentals of two-phase heat transfer (boiling and condensation) taking place inside multi-channel heat pipes, a unique three-leg heat pipe has been built. This one-of-a-kind heat pipe was used to develop both computational fluid dynamic (CFD) and theoretical models of a multi-channel heat pipe. To simulate the heat pipe operation with ANSYS Fluent, the Volume of Fluid (VOF) approach and Lee model were investigated. Different types of Lee models using user defined function (UDF) were compared and the influence of the condenser's boundary condition, saturation temperature, and mass transfer coefficient on the simulations was studied. For the first time, major limits of the Lee model for the simulation of heat pipes are identified. It is concluded that the available Lee model cannot predict the heat pipe temperature as it shows low physical meaning and can easily be manipulated to adjust the simulation's results. Based on the three-leg heat pipe experimental data, a new multi-channel theoretical model was developed that uses the thermal-electrical resistance analogy to predict the three-leg heat pipe thermal resistance. By selecting the optimum correlations for pool boiling and filmwise condensation, the developed iterative theoretical model was able to predict the three-leg heat pipe thermal resistance with an error of 8.2%. European Union's H2020 Programme ETEKINA and iWAYS under grant agreement numbers 768772 and 958274.

In the present paper, a fundamental analysis on the effect of the channel aspect ratio on the bubble dynamics and heat transfer characteristics for the early transient stages of the bubble growth within confined microchannels of rectangular cross-section, under saturated flow boiling conditions, is conducted, utilising high resolution, 3D, transient, conjugate heat transfer simulations. The open-source toolbox OpenFOAM is applied for the simulations, utilising a custom, user-enhanced, diabatic Volume OF Fluid (VOF) solver. Two different series of numerical simulations are performed, focused on a single nucleation event from a single nucleation site and a single nucleation event from multiple, arbitrarily located, nucleation sites, respectively. In each series, three different values of channel aspect ratio are considered, corresponding to a narrow, a square, and a wide microchannel. For the first series, the simulations are performed for a low, a medium, and a high value of applied heat flux and mass flux. For the second series, only the lower values of applied heat flux and mass flux are used for each channel aspect ratio, since this constitutes the worst-case scenario from the overall heat transfer performance point of view, amongst the cases examined in the first series of simulations. The micro-passage aspect ratio has a significant effect in the generated bubble dynamics during the onset of the nucleate boiling regime, as the bubbles grow within the confined liquid crossflow. This alteration of the generated interfacial dynamics, in effect, regulates the size and position of the contact areas of the generated bubbles with the microchannel walls, with a direct effect in the individual contribution and therefore, the balance between the contact line and the liquid film evaporation mechanisms. Moreover, the work presents the quantification of the effect of the solid domain thermal inertia on the whole process and in particular on the local Nusselt numbers. It is evident that considering conjugate heat transfer in numerical simulations of flow boiling is compulsory in order to predict the physical processes in a correct form.

Heat transfer measurements of flow boiling Carbon Dioxide (CO$_2$) in vertical upward direction have been carried out with a dedicated test facility at the European Organization for Nuclear Research (CERN). The investigation covers saturation temperatures within a range of $-25 \degree \textrm{C} \leq T_{\mathrm{sat}} +5 \degree \textrm{C}$, mass velocities from 100kg m$^{-2}$ s$^{-1}$ $\leq G \leq $ 450 kg m$^{-2}$ s$^{-1}$ and heat fluxes of 5.3 kW m$^{-2}$ and 11.4 kW m$^{-2}$. The experiments have been conducted in a vertical upward evaporator of 8 mm inner diameter and 8 m length and a database of more than 1900 measurements has been compiled within the present study. The heat transfer mechanism is dominated by the nucleate boiling contribution and dryout is observed as a function of saturation temperature, mass velocity and heat flux. A correlation is proposed to predict the dryout inception within the experimental range where the onset of dryout has been observed. The results suggest that most commonly used heat transfer prediction models underpredict the heat transfer mechanisms of CO$_2$. Moreover, the heat transfer coefficients of CO$_2$ increase in vertical upward direction, compared to the data of horizontal studies. For that reason, a vertical multiplier is suggested to capture the trends of vertical upflow with two existing prediction models.

This paper reports on a parametric study of the impingement heat transfer from swirling jets issuing from a circular nozzle. The swirl velocity component is imparted to the flow via a tangential entry into a recirculation chamber located upstream of the nozzle; the degree of swirl is varied by changing the angle of the channels leading to the chamber with respect to the radial direction. The effects of the dimensionless impingement distance (H/D = 1, 2, 3, 4, 6, 8, 10, 12, 14 with D being the nozzle diameter), the swirl number (S = 0, 0 . 078, 0 . 22, 0 . 37, 0 . 54) and the Reynolds number (Re = 20,800 , 31,100 , 41,500) on the spatial distribution of the heat transfer are investigated by means of infrared thermography and the heated thin foil sensor. The present results show that the swirl number and the impingement distance significantly influence the structure of the heat transfer distribution, while the Reynolds number affects essentially the magnitude. Four distinct ranges with different behaviours can be identified by distinguishing between weakly or non swirling jets (0 ≤S ≤0 . 22) and moderately swirling jets (0 . 37 ≤S ≤0 . 54) and between short and long impingement distances (1 ≤H/D ≤4 and 6 ≤H/D ≤14 respectively). A comparative assessment of the performance of the swirling jets is carried out by focusing on the area-averaged Nusselt number distributions. It is found that in the range of short impingement distances and over relatively small target areas an enhancement of the heat transfer rates can be obtained at a small extent by adding a weak swirl with no detrimental effect on the uniformity of the distribution and at a larger extent by a moderate swirl with a deterioration of the uniformity. For long impingement distances no enhancement is observed, although swirl yields a significant reduction of the non-uniformity. Finally, correlation laws of the area-averaged Nusselt number as a function of the control parameters are derived in the four regimes identified.

In this paper, the improved incompressible smoothed particle hydrodynamics (ISPH) method is used to conduct a numerical simulation for the buoyancy driven flow inside an enclosure including a cross shape that is filled with moving/fixed solid particles. In this study, a Cu-water nanofluid is assumed as a working fluid. The sidewalls of the cavity are cooled, the horizontal cavity walls are adiabatic and the cross shape with the inner solid particles were heated. Different cases are taken into account based on the temperature conditions on the inner solid particles, namely, fixed and cold solid particles, moving and cold solid particles, fixed and hot solid particles and moving and hot solid particles. The controlling parameters in this study are dimensions of the cross shape L cross , depth of the solid particles L solid , the Rayleigh number Ra and the nanoparticles volume fraction ϕ . The main results revealed that the decrease in the cross shape lengths by 0.6 increases values of the stream function by 27.8% and the isotherms lines are enhanced. In addition, the case of the cold and moving solid particles gives the higher rate of the heat transfer while the lowest rate of the heat transfer is observed in case of fixed and cold solid particles.

This paper presents a mathematical deduction of a new improved model for heat transfer during condensation inside tubes. This new model has been developed with the aid of the Gaussian Equation for an infinite straight line, considering this relation with the differential equations that govern the heat transfer process. The proposed model was verified by comparison with available experimental data of 22 different fluids, including various refrigerants, water, and organic substances, which condense inside vertical, inclined and horizontal tubes. The proposed model is valid for an reduced pressure values ranging from 0.0008 to 0.91, values of Reynolds number for single-phase between 68 and 84,827 and for Reynolds number for two-phase between 900 and 59,4373, a range of internal diameters ranging from 2 to 50 mm, vapor quality from 0.01 to 0.99, P r values for single-phase from 1 to 18 and mass flux rates in the ranges of 3 to 850 kg/(m 2 s). The mean deviation found for the analyzed data for horizontal tubes was 11.8%, while for the vertical and inclined tubes data the mean deviation was 13.0%. In all cases, the agreement of the proposed model is good enough to be considered satisfactory for practical design.

A new type of three-dimensional pin–fin plate with elliptical cross-section was proposed. Condensation heat transfer of FC-72 in a narrow rectangular channel with the proposed pin fins was experimentally studied in normal gravity and microgravity. The effects of pin geometry, thermal conductivity, mass flow rate and gravity on condensation heat transfer were investigated. The visualization experiment under microgravity was carried out and the condensate behaviour was observed by high CCD camera. The results showed that all the elliptical pin-fin plates exhibited substantially better performance than the flat plate. The influence of pin geometry and thermal conductivity on condensation heat transfer coefficient was great. The smaller pin size and higher thermal conductivity were favourable for condensation enhancement. The average condensation heat transfer coefficient increased with the increase of mass flow rate. In microgravity, obvious fluctuations and climbs occurred in gas-liquid interface. However, the condensate behaviour on the condensing surface was unconspicuous. For unsteady state, the vapor-side temperature difference increased evidently in microgravity. Microgravity had a certain effect on temperature evolution of the condensing base. Short-term microgravity degraded the condensation heat transfer coefficient in quasi-steady state or unsteady state, but had little effect on pulsating state.

Deep borehole heat exchangers (DBHE) provide an effective solution for ground coupled heat pump (GCHP) systems in cold climate region where heating is dominant. Concerning the analytical heat transfer models, the simplification that borehole wall temperature is constant along the depth of ground heat exchangers (GHE) which the existing quasi-three-dimensional models have assumed in the application of shallow borehole GHE, can no longer be accepted in the application of DBHE due to the geothermal gradient in deep ground. Making this simplification cannot give the real temperature distribution of circulating fluid along the depth of DBHE. Therefore, this paper developed a new analytical model for DBHE with coaxial pipes by successfully addressing the increasing borehole wall temperature using the convolution theorem, so that the widely employed quasi-three-dimensional models for shallow borehole GHE is extended for DBHE with coaxial pipes for the first time. The new analytical model is validated by comparing with an existing numerical model. Using the newly developed analytical model, the trends revealing the relationships between thermal performance of DBHE and various parameters are firstly plotted. Because of the high accuracy and quick calculation, this new analytical model can be used as a benchmark for numerical models. More importantly, the proposed analytical model can be an effective tool for the design and optimization of DBHE, since current numerical models are always calculation-demanding, time-consuming and difficult for engineers and designers to use. Also, the method this paper proposed to address the varying borehole wall temperature can certainly be employed to improve the existing quasi-three-dimensional models for shallow borehole GHE so that they can be applicable in some other cases, for example, GHE installed in layered soils, or affected by underground seepage flow in partial depth.

Experimental and numerical studies on the effect of heat transfer to an open cavity placed in supersonic crossflow are presented. Cavity floor is subjected to the desired amount of heat flux and the consequent effects in the flow dynamics inside the cavity and its neighborhood are systematically assessed. Cavity flowfield has been visualized using high-speed Schlieren imaging and pressure on the cavity floor has been measured using both steady and unsteady pressure transducers. Two-dimensional unsteady compressible turbulent flow field has been numerically simulated using a Harten-Lax-van Leer-Contact (HLLC) scheme based unstructured finite volume method solver. The numerical method has been validated using both experimental data available in the literature as well as the wall pressure measured in the present study. Significant changes in flow dynamics such as the growth of the recirculation region, shearlayer oscillation, shearlayer impingement on the aft wall of the cavity, and frequency of cavity induced oscillations have been observed. Experimental observations are well complemented by the numerical studies and could reveal the physics of alteration in cavity flow dynamics with the heat addition.

Ice slurry is attracting increasing attention owing to its high cooling rate, heat transfer capability and energy storage density. This paper provides a state-of-the-art review of ice slurry flow and heat transfer characterization in typical components of cooling systems including pipes, fittings, valves, pumps and heat exchangers by experimental measurements, analytical quantifications and numerical simulations. Generally, the experimental measurements concerned the flow pattern, pressure drop, rheology and heat transfer of ice slurry, and the measurement data were usually used to develop and validate analytical and numerical models. The analytical quantification was commonly performed by the principles of non-Newtonian fluid mechanics, which simplified the multiphase flow into a single-phase flow and can predict the friction factors and heat transfer coefficients. The numerical simulation employed multiphase flow dynamics to establish the governing equations for ice particles and carrier fluids in ice slurry flow and heat transfer with phase change, thus providing predictions on different physical fields of ice slurry flow. It indicated that the experimental measurement, analytical quantification and numerical simulation presented different advantages and disadvantages in describing ice slurry flow and heat transfer. The combination of numerical simulation with the experiment measurement and analytical quantification is promising to improve the performance of each single method in terms of accuracy, efficiency and generality. Moreover, the development of optimization strategies to maximize the performance of cooling system with ice slurry is essential.

Heat transfer performances of the impinging jet arrays issued from the nozzle plates without and with the two types of the interconnected grooves were experimentally studied at jet Reynolds numbers ( Re j ) and nozzle-to-plate distances (S) in the respective ranges of 1500–20,000 and 0.1–8 times of the nozzle diameters ( d j ). Owing to the significant spent flow confinement formulated between the smooth nozzle plate and the impinging surface at S/ d j d j effectively resumed the HTE characteristic in association with the impinging jet array. At large S/ d j , the additive ambient fluid entrainment from the grooves enriched the jet momentum to boost the heat transfer level from that produced by the smooth nozzle plate. Both the Nusselt number level and the heat transfer uniformity for the smooth and grooved nozzle plates were comparatively studied with the Re j and S/ d j effects examined. In conformity with the experimental observations for the isolated and coupled Re j and S/ d j effects, the regression-type analysis was performed to generate a set of empirical correlations that permitted the evaluation of the average Nusselt number ( Nu ) over the central jet region for each of the present impinging jet arrays.

The molten fuel is first discharged into a drain tank as the molten salt reactor is shut down. Some heat transfer tubes need to be placed in the drain tank accounting for the generation of decay heat by the fission products. The tube of special structure has no need to pass through the entire drain tank. It’s a two-layer assembly where a center tube is inserted in a thimble. A single prototype tube without scaling is studied in this paper. The results show that the absorption ratio by the center tube is approximately unchanged. There is a transition zone in the nucleate boiling region before the linear change of superheat and heat flux. The ONB data was used for the validation of the models and an improved correlation was developed.

Described in this paper is a numerical investigation on the concept for improving thermal performance of internal cooling by placing the convergent and divergent slit ribs. Five different geometrical models are investigated, including the ribs of rectangular slits and trapezoidal slits with different convergent and divergent angles. The effects of slit shape and its convergent or divergent angles on thermal performance of internal cooling are evaluated with the Reynolds numbers ranging from 10,000 to 25,000. Results obtained show that the turbulence intensity in cases with the smallest angle trapezoidal slits are in the highest level, which produces the highest level of heat transfer enhancement and highest level of pressure loss. The thermal performance index, which comprehensively evaluates the thermal performance of internal cooling, shows that the thermal performance in cases with the smallest angle trapezoidal slits are in the highest level due to the increased heat transfer enhancement and limited increase of pressure loss.

Nanoemulsion absorbents are manufactured using methanol and undecane for CO2 absorption and regeneration performance enhancement. The most representative physical CO2 absorbent, methanol, is mixed with undecane and dispersed by the ultrasonication method. Span 60 and Tween 60 are used to obtain a high dispersion stability. To determine the effect of undecane concentration on the combined heat and mass transfer performance during the absorption and regeneration processes, various concentrations of nanoemulsion absorbents are prepared. The thermal conductivity of the nanoemulsion is also measured to estimate the effect of nanodroplets on the heat transfer performance. It is concluded that the average absorption rate increases by 13.04% at 0.05 vol% of undecane and the average regeneration rate does by 22.03% at 0.05 vol% compared to the base absorbent. Various mechanisms for combined heat and mass transfer performance enhancement by nanoemulsion absorbents are discussed. It is concluded that the mechanism of regeneration performance enhancement by the thermal effect in the nanoemulsion absorbents is less effective compared to that in the nanofluid absorbents.

Mitigation of fouling and particulate deposition in heat exchangers is the foremost concern of this critical review study. Formation of vindictive scale, sludge and fouling layers on heat transfer surfaces have severe influence on thermo-hydraulic performance and overall efficiency of heat exchangers. Fouling on gas or liquid side prevent heat transfer from bulk fluid to the solid surfaces resulting lower thermal performance of heat exchangers. Furthermore, fouling layers on heat transfer surfaces induce blockage to the fluid flow and hence increase pumping power by amplifying pressure drop. The use of various system operating variables in controlling fouling and particulate deposition is comprehensively discussed. The influence of flow rate or velocity, temperature, concentration, material surfaces and other miscellaneous factors on reduction of fouling and particle deposition is extensively investigated. It was concluded that higher flow velocity induces strong shear force across heat transfer surface which in results eradicate deposits and reduces the fouling resistance at the expense of higher pressure drop. The enhancement in fouling concentration leads to the higher fouling resistance and materials with lower thermal conductivity yield inferior fouling resistance. To conclude, this review study will be exceedingly useful for designers to design heat exchangers with higher overall efficiency under the influence of fouling.

In this study, an experiment on condensation heat transfer was conducted to develop a condensation model considering the structure of separated flow patterns. Multidimensional local condensation heat transfer parameters were measured in pure saturated steam at pressures of 1–5 bar and mass fluxes of 10–50 kg/m2 s in an inclinable tube with an inner diameter of 40 mm and a length of 3 m. A heat partition angle, which separated the upper and lower heat transfer areas for film condensation and convective heat transfer in stratified flow, was obtained based on the inflection point of the circumferential distribution function of local heat flux. A new condensation heat transfer model consisting of the heat partition angle and heat transfer coefficient correlations was developed based on the local heat transfer data. The experimental data for model development were obtained using circular tubes with inner diameters of 30–45 mm and inclination angles of 0–10° under pure saturated steam conditions at pressures of 1–67 bar and mass fluxes of 10–329 kg/m2 s. The model predicted the average heat transfer coefficient with an average deviation of 6.2% against the present experimental data.

The various influence factors of macrosegregation are complex and have been researched widely due to their undesirable effect on continuous casting slab. Based on an Eulerian approach, a multiphase solidification model combining turbulent fluid flow, heat transfer, microstructure evolution, solute transport with back diffusion and shell deformation were developed in this work to investigate the fluid flow and macrosegregation in continuous casting slabs under the effects of shell bulging and mechanical reduction. In this model, five phases of the slab were considered: the liquid phase, inter-dendritic melt phase of equiaxed grains, solid phase of equiaxed grains, inter-dendritic melt phase of columnar dendrites, and solid phase of columnar dendrites. The predicted temperature, shell thickness and solute element distribution were verified by the results of thermal infrared imaging, nail-shooting experiments, macrostructure analysis, and carbon-sulphur analysis. In this model, the asymmetrical bulging between two adjacent supporting rollers was considered, and its effect on the fluid flow and macrosegregation of the slab was investigated. The calculation results show that the positive centreline segregation considering the asymmetrical bulging profiles was more serious than that considering the regular sinusoidal shell profiles. Using this model, the slab macrosegregation was investigated with different reduction mechanisms in the mushy zone; a large reduction applied just before the solidification end could significantly reverse the flow of solute-enriched melt and reduce the macrosegregation. These results were also verified by an industrial application.

This study introduces Kagome lattice structures to replace pin fins or ribs in a wedge-shaped channel, which represents a turbine-blade trailing edge, for heat transfer enhancement. Present simulation methods are verified against available experimental data. The local and overall thermo-fluidic characteristics of the four designed channels, at five Reynolds numbers and a given porosity, are investigated numerically and compared. The results reveal that the proposed structures exhibit 6–71% higher overall Nusselt numbers relative to the original structure but with similar pressure loss. The heat transfer enhancement is attributed to the fact that the first array of Kagome cores separates more high momentum fluid to the tip region, which results in the reduction of recirculation and increases convective heat transfer.

Vortex generation has emerged as a promising passive method for enhancing air-side heat transfer. In the present work, two-dimensional numerical analysis is carried out to examine the thermo-fluid analysis of fin-and-tube compact heat exchangers with sinusoidal wavy and elliptical curved type rectangular winglet vortex generator (RWVG) having varying Reynolds number ranges from 400 to 1000. The wavy and curved VGs are also placed as up and down configurations with respect to flow direction. Thus the effect of the different common flow down (CFD) configurations of wavy-up, wavy-down, curved-up, curved-down and flat rectangular winglet is compared with baseline configuration (non-winglet case) for flow structure, temperature distribution and pressure distribution for fin-and-tube compact heat exchanger with seven inline circular tube arrangement. They are also compared with thermo-hydraulic performance criterions which include Nusselt number (Nu), Pressure drop (ΔP), friction factor (f), London area goodness factor (j/f). Appreciable improvement in heat transfer characteristics is observed with wavy and curved rectangular winglet with a moderate loss in pressure. Compared with the non-winglet baseline and flat RWVG case, the heat transfer performance of the fin-and-tube heat exchanger is significantly improved with wavy-up and down and curved-up and down configurations. Further, it is also noted that up-wavy RWVG showed the best heat transfer improvement compared to any other RWVG configuration considered. However, Wavy-up configuration has the lowest values of j/f than others and curved-down has higher values of j/f than others. Thus, curved-down RWVG found suitable than other cases of RWVG as far as London goodness factor strategy.

The aim of this work is to improve numerical predictions of natural convection heat transfer inside of gas turbines. It is desirable for engineers to use numerical simulations be able to accurately predict the heat transfer inside of a gas turbine casing during engine shutdown. This will help them to understand and alleviate problems such as engine casing distortion. A natural convection test rig was constructed to provide simulation validation data. The rig represented a section of a large gas turbine casing and operated at the same Rayleigh number as a real engine. The casing rig was able to provide a unique dataset that could not be obtained through real engine measurements. Analysis of the casing rig was performed using a large eddy simulation and RANS simulations. It was found that the baseline RANS simulation did not predict the heat transfer accurately enough for engineering use. The simple gradient diffusion hypothesis (SGDH) turbulent heat flux model used in the baseline simulations was replaced by more advanced generalised gradient diffusion hypothesis (GGDH). A modification to the GGDH model, called GGDH+, was developed to account for buoyancy effects on the turbulent heat flux. The GGDH+ model was then able to give heat transfer predictions comparable to the LES results.

Sodium carboxymethyl cellulose (SCMC) is one of the most promising scale inhibitors. In this study, the performance of SCMC in inhibiting the deposition of calcium carbonate on heat transfer surface at the temperatures ranging from 293 to 343 K was investigated by experiments, and the interaction between SCMC and calcite in the solution was explored by molecular dynamics simulation. The experimental results showed that the fouling resistance substantially decreases by adding the SCMC. The scale inhibition efficiency increases with the solution temperature, and the highest scale inhibition efficiency at 99.8% at 343 K. The simulated results showed that with the increase of the solution temperature, both the binding energy between SCMC and calcite plane and the probability of SCMC binding with calcium ions increase, which can effectively prevent the growth of calcium carbonate fouling on the heat transfer surface. Therefore, our computational results agree with the anti-scaling experimental phenomena.

This study explores the influence of counter current airflow on evaporative heat transfer performance of falling films on a horizontal elliptical tube. The numerical analysis was based on the volume-of-fluid simulations to catch the dispersal evolution of descending films in conjunction with user defined function to model the evaporation effect at the water surface to the ambiance. The calculated results of heat transfer coefficients were in reasonably good agreement with the measured data from the literature for validation of the computational model. Calculations were performed to elaborate the interaction mechanism of counter current airflows with the formation process and heat transfer of evaporative liquid films on the tubes. In essence, upward air streams can induce the shear stresses at the water-air interface to form thicker wavy films. The counter current airflow has a more pronounced effect on heat transfer enhancement in the lower part of the tube than the upper part. Numerical experiments were also extended to assess the thermal performance of water film evaporators by systematically varying the inlet liquid mass flow rate of 0.093–0.186 kg/m-s and counter current airflow velocity of 0–3 m/s, respectively. The simulations results reveal steeper temperature gradients at the wall to enhance heat transfer outcomes, achieving an averaged heat transfer coefficient of 4.15 kW/m2 K for Γ = 0.149 kg/m-s and Vair = 3 m/s.

The special topology of the X-lattice induces a large-scale primary spiral flow and three kinds of secondary flow skewed towards and away from the endwall regions and behind each ligament. To make full use of secondary flows produced in different regions, dimples, protrusions and pin fins are separately introduced to the endwall of a sandwich panel to further improve the overall cooling performance. Forced air convection in the six sandwich panels is numerically investigated based on a validation of the numerical model against experimental data available in the literature. Results reveal that the case with pin fins installed in the region upstream of the crosses in the X-lattice (Region A) exhibits the best overall heat transfer performance, while the case with dimples installed downstream of the crosses in the X-lattice (Region B) shows the poorest heat transfer. In terms of the pressure drop, the introduction of protrusions or pin fins in Region A slightly decreases the pressure drop within a certain range of Reynolds number, but the other four cases show an increase in pressure drop. Taking the pressure drop and heat transfer into consideration, a proper combination of the X-lattice and the elements can enhance the overall thermal performance of the sandwich panel. The modifications of the fluid flow and local heat transfer characteristics by the added elements are well analyzed to understand the underlying mechanisms.

A conjugate heat transfer simulation of transient turbulent flow in a scaled turbine valve, which usually occurs in the fast start-up processes of coal-fired power plants, was performed against experimental validation. A high-temperature (615 °C) experimental system with a scaled (1:3) turbine valve was set up at Pennsylvania State University. Eighty thermocouples were flush-mounted in streamwise and circumferential directions inside the valve body, and spatio-temporally varying temperature and temperature gradients were acquired as the mainstream temperature and pressure rapidly varied. A simulation using the shear stress transport model showed considerably better agreement with the measured temperature than the standard k - e model and the Realizable k - e model; the numerical errors in valve top, valve chamber, valve seat and valve diffuser were below 1%, 2%, 5% and 8%, respectively. However, the largest errors located in the upper diffuser were confirmed to be associated with alternating oscillations of the annular attachment jet along the diffuser surfaces. Further investigations of transient thermal behaviors demonstrated that the instability of large-scale vortical structures inside the valve diffuser significantly enhanced heat transfer between the valve body and the air flow. In addition, upstream straighteners enabled the formation of separated secondary flow structures inside the diffuser, resulting in non-uniform heat transfer along the valve’s circumferential direction.

The three–phase gas–liquid–solid flow, caused by hydrate decomposition in cuttings is a main concern during drilling through gas–hydrate reservoir. In this study, a transient gas–liquid–solid flow model is developed considering the coupling interactions between hydrate dynamic decomposition, cuttings transport and heat transfer in multiphase flow. Using this model, the transient gas–liquid–solid flow behaviors are investigated. Numerical simulations show that the decomposition rate of hydrate in formation is only 1/140 of that in annular cuttings for a unit depth, therefore, the influences of hydrate decomposition in hydrate layers can be neglected. Hydrate particles undergo three processes from bottom hole to wellhead in annulus: non–decomposition, slow decomposition and rapid decomposition. In annulus where the depth is more than 400 m, hydrates decompose slowly and the decomposed gas hardly expands due to the high pressure. While, if the hydrates and decomposed gas return upwards to the position where the depth less than 400 m, the gas void fraction increases significantly, not only due to the faster decomposition rate of hydrates but also due to the more intense expansion of decomposed gas. After the hydrate particles return upwards to the wellhead, the behaviors of gas–liquid–solid flow tend to be a quasi–stable state. If there is no backpressure device at the wellhead, that is, the wellhead backpressure is 0 MPa, the gas void fraction at the wellhead can reach 0.68, which is enough to cause blowout accident. Increasing wellhead backpressure to 2 MPa through managed pressure devices and lowering the inlet temperature of drilling fluid to 17.5 °C except adjusting drilling fluid density can manage the gas void fraction within 10%.

The present study initially considered the double wavy (DW) shape as the geometric disturbance to control fluid flow and the heat transfer. The large eddy simulation (LES) was performed to solve the filtered momentum and energy equations at the subcritical Reynolds number of 3000 and the Prandtl number of 0.7. In order to evaluate the effect of DW geometry on the force coefficients and the heat transfer, the smooth (CY), and symmetric wavy (SW) and asymmetric wavy (ASW) cylinders are considered for the purpose of the comparison. The DW cylinder achieved the smallest mean drag and the largest suppression of the lift fluctuation among the different geometric disturbances. The Nusselt number of the DW cylinder has the smallest mean and fluctuation values. The spanwise local Nusselt number is strongly correlated to the geometric disturbance, which is consistent with the SW and ASW cylinders. The DW cylinder provided smaller spanwise local Nusselt number over the span than the SW and ASW cylinders. The DW cylinder attenuates the heat transfer in comparison with the circular cylinder. The wake modification by the DW cylinder associates to the attenuation of the heat transfer.

This paper reports heat transfer enhancement techniques for the cryogenic quenching process. An experiment was performed to evaluate the enhancement of quenching heat transfer by two techniques: 1. Using intermittent pulse flows and 2. Coating the inner surface of a test tube with layers of low thermal conductivity films. Pulsed liquid nitrogen flows with various duty cycles and periods were applied in the quenching of a room-temperature metal tube coated with four layers of Teflon thin films on the inner surface. In general, the results obtained indicate that the quenching thermal efficiency that measures the effectiveness of heat transfer enhancement increases with decreasing duty cycle, however it is relatively independent of the period. Comparing with non-coated bare surface test tube, the low-conductivity coating substantially improved the thermal efficiency and reduced the total quenching time. Additionally, the thermal efficiency was found to increase with decreasing source inlet pressure or coolant mass flow rate. The savings on the amount of cryogen consumed follow the same trends as those for the thermal efficiency with pulse flows and tube coating.

In this paper we analyse the jet of non-Newtonian power law fluids emerging from a tube with a diameter D and impinging on a flat plate, which is receiving from the other side a constant heat flux. To that end, different numerical simulations have been carried out with the one dimensional fully developed axisymmetric velocity profile used as boundary condition to model the jet. The aim of the work is to quantify the heat (through the Nusselt number) and momentum (through the friction coefficient on the plate) transfer processes between the jet and the plate as a function of the Reynolds number ( Re = 50 , 100 , 200 ), the tube-to-plate distance ( H / D = 1 , 2 , 4 ) and the power law index ( n = 0.8 , 0.9 , 1.0 , 1.1 , 1.2 ). The values given to the power law index allowed to explore both the shear thickening and the shear thinning behaviours. Results showed that a larger tube-to-plate separation decreases both the transference of heat and momentum; additionally, increasing the Reynolds number improves the heat transfer in detriment of momentum transfer; and, similarly to this latter input parameter, lowering the power law index, increases the heat transfer and worsens the momentum transfer. These results are summarised in three mathematical correlations consisting of potential functions of the input parameters, i.e. Re , H / D and n. However, as it is usually done, the Prandtl number Pr will be used instead of n.

Pool boiling performance of NOVEC-649 was experimentally studied on microporous surfaces prepared by an electrochemical deposition method. Microporous structures contribute to large surface roughness values and provide large quantities of cavities ranging from several hundreds of nanometers to several microns for bubble nucleation. The results show that a maximum enhancement of 600% in heat transfer coefficient and a maximum enhancement of 55% in critical heat flux are achieved on the deposited surfaces, compared with a smooth copper surface. Experimental heat transfer coefficients were compared with literature correlations, considering the effects of roughness and surface-liquid combination. Then a fitted Rohsenow correlation was discussed and developed to predict the present results. Experimental critical heat fluxes were compared with classical models. It was found that the critical heat flux on the smooth surface could be predicted by the lift-off model and the Kandlikar model, but these models cannot predict the critical heat fluxes on the deposited surfaces well. Following, the Kandlikar model was modified by further considering a wicking force and a roughness-factor-dependent surface tension force. The present modified CHF model was validated by comparing with present experimental data and the literature, with a deviation around ±30%.

This paper numerically investigates the effects of the film cooling hole diameter on the flow and heat transfer characteristics of the impingement/effusion cooling and double swirl/effusion cooling. Impingement jet arrays at three jet Reynolds numbers, 10,000, 15,000, 20,000, are employed. The target channel consists of a semicircular channel in impingement/effusion cooling and two partially overlapping cylinders in double swirl/effusion cooling. Three arrays of film cooling hole rows are established on the target surface under two arrangements. Four film cooling hole diameters, 0.4, 0.6, 0.8 and 1.0 times the jet hole diameter, are considered. The flow structure and flow development inside the target channel are compared and analysed. The heat transfer performance are discussed and compared. Results show that the effusion air distribution and Nusselt number distribution is more uniform in double swirl/effusion cooling. With the application of the double swirl channel, about 20–33% increase in overall averaged Nusselt number of the whole target channel and about 12–20% increase in spatially averaged Nusselt number at the effective cooling region are obtained. With the application of the film cooling holes, the maximum increase in spatially averaged Nusselt number at the effective cooling region is 10.3% in impingement/effusion cooling and 4.7% in double swirl/effusion cooling.

Mushy zone refers to the two-phase mixed region appearing between the solid and liquid region during solid-liquid phase change. The microstructural evolution within mushy zone is tightly coupled with the fluid flow, heat transfer and phase transition, as well as the related latent heat storage performance. In this paper, the microscopic structural evolution during paraffin’s melting and solidification was observed in real time using a confocal optical microscope equipped with a thermal stage. Based on image postprocessing, the evolving features of the mushy zone are quantified, including the characteristic length of the interphase liquid, solid/liquid fraction, solid/liquid formation rate, and latent heat evolution. Our results indicate that three regions, i.e. the solid region, the mushy zone and the liquid region, can be distinctly identified during both the melting and solidification processes. Meanwhile, it is found that the microstructure within mushy zone during melting evolves quite differently from that in solidification process, implying that the former is not just a simple reverse process of the latter. In addition, the mushy zone constant, coming from the enthalpy-porosity method that simulates the macroscale heat transfer of solid-liquid phase change, was evaluated according to the measured microstructure feature. These results provide physical insights into the nature of mushy zone during both the melting and solidification processes, and also offer instructive guidelines for accurate modelling of macroscale solid-liquid phase change heat transfer.

This research presents the outputs for nano-refrigerant (R600a/oil/CuO) boiling heat transfer within flattened channels utilizing experimental method. The influence of flattened percentage, flow rate, vapor quality as well as the mass fraction of CuO on boiling heat transfer (h) were discussed. Outcomes reveal that increasing the flattened percentage enhances the h. Also, within the ranges of present experiment, h augments by increasing nanoparticle’s concentration.

This paper presents a novel numerical framework based on the accurate spectral element method (SEM) for electro-thermo-hydrodynamics (ETHD), in which the coupled electric, flow, and heat fields are solved simultaneously. The electric field potential is estimated by the classic Galerkin variation with the electric field vectors evaluated by the spectral approximation, and the streamline upwind Petrov-Galerkin variation and consistent approximate upwind technique are applied to stabilize the numerical simulation of charge density transport. As for the flow and heat transfer, the improved time-splitting method developed in our previous work is applied to deal with the coupled pressure and velocity in the momentum equations. Three carefully selected cases of analytical solution, pure thermal and pure electric convection, and electro-thermo-convection are performed to validate the algorithm. Then, the electro-thermo-convection in an enclosure with uniform temperature distribution is simulated and the average Nusselt numbers at different electric Rayleigh numbers are presented accurately. Additionally, the sinusoidal temperature boundary conditions are considered that the contours for stream function, temperature, and charge density are analyzed in detail to depict the heat transfer enhancement with ETHD effects.

Results of an extensive experimental investigation performed to study the effect of different values of wall heat flux in a rectangular PCM (phase change material) test cell on the melting process are presented. A new experimental system consisting of a rectangular cross-section test cell formed from polycarbonate sheet, copper plates and mica heaters was constructed. During experiments uniform wall heat flux (q″wall = 675, 960 and 1295 W/m2) were applied to both the left and right sides of the test cell. Thermocouples were used to measure the temperature at different locations inside the PCM and on the surface of the copper plates and an infrared camera was used to measure the polycarbonate sheet external surface temperature distribution. The results show the expected strong correlation between the magnitude of wall heat flux and the melt fraction in the PCM as it drives the convective heat transfer. The transparent polycarbonate wall makes it possible to observe the location of the solid/liquid interface and determine melt fractions. The experiments have produced a significant experimental data set for the validation of numerical models simulating the solid/liquid phase change process and PCM melting in geometrical configurations relevant to, for example, latent heat thermal energy storage systems.

Using the Eulerian-Eulerian two-fluid model (TFM) the hydrodynamics and heat transfer around different immersed bodies in a fluidized bed were analyzed. The kinetic theory of granular flow (KTGF) was used to simulate the solid phase. For hydrodynamic simulations, 3D, 2D Cartesian and 2D axisymmetric frameworks were examined and the differences in the results of these simulations were discussed. For the heat transfer analysis, due to the high computational demand of 3D simulations, the analyses were performed using 2D Cartesian and axisymmetric frameworks. For studying the effect of shape on the CFD results, two cases of spherical and cylindrical immersed bodies were simulated. In addition, two methods for calculating the surface-to-bed heat transfer coefficient (HTC) for 2D Cartesian and axisymmetric models were examined. The first (method I) is based on constant heat flux boundary condition, while the second one (method II) is based on the isothermal wall boundary condition. It was found that method I outperforms the second one for both 2D Cartesian and axisymmetric configurations in prediction of average HTC. It was shown that the simulation results were in closed agreement with the corresponding measured data. The findings revealed that the spherical immersed body, which has a better aerodynamic shape, produced higher HTC in bubbling fluidized bed with Geldart B particles. Finally, the impact of Archimedes number (Ar) on the surface-to-bed HTC was studied.

Experimental measurements were performed to study the effects of vortex generator (VG) height H/D = 0, 0.37, 0.75, 1.00, 1.27, 1.50 and location L/D = 1, 2, 3, 4 on VG enhancing the film cooling performance at different blowing ratios (M = 0.5, 1.0, 1.5) in a flat plate. Detailed velocity field and streamwise vorticity distribution visualized by Particle Image Velocimetry (PIV) system showed that an anti-counter-rotating vortex pair (ACRVP) was generated by VG, whose strength and size increases no matter how M or H/D increases. The film cooling effectiveness distribution measurements by infrared camera indicated that VG shows a great advantage in enhancing film cooling. Especially at M = 1.5, the area averaged film cooling effectiveness of the model with H/D = 1.00 could increase by 220%, compared with that of the model without VG. As H/D increases, the film cooling performance becomes better and then worse, because the increase of VG height helps entrain more injection flow for the short VG but the mainstream would be entrained into the strong ACRVP towards the wall when VG is too high. Besides, as M increases, the film cooling performance of models with H/D = 0, 0.37 becomes worse while those models with H/D = 1.0, 1.27, 1.50 becomes better. For the impact of VG location, measurements conducted on the model with H/D = 1.00 at M = 0.5, 1.0, 1.5 shows that VG location hardly affects the film cooling attachment except for the zone around the VG. At the high blowing ratio M = 1.0 and M = 1.5, the film cooling effectiveness on the zone around VG decreases significantly when the VG location varies from L/D = 1 to L/D = 4, but at M = 0.5, it hardly changes with the VG location varying.

Nanostructured surfaces present great potentials to enhance nucleate boiling heat transfer. However, due to the diverse materials and approaches of fabrication, nanostructured surfaces have a wide range of micro/nano-morphologies and different or even conflicting effects on the critical heat flux (CHF) and heat transfer coefficient (HTC), causing serious uncertainties to the design of optimal nanostructures for thermal management. The mechanisms behind the uncertainties are yet to be fully understood. This paper presents a comprehensive review of the nanostructures, bubble dynamics phenomena and boiling heat transfer performance of various nanoengineered surfaces. It is proposed that the evaporation of liquid menisci in nanoscale pores triggers a significant negative pressure in the porous structures, which causes an additional heat and mass transfer mechanism and significantly changes the features of bubble nucleation, growth and departure, as well as the CHF and HTC. However, the effectiveness of the negative pressure is highly sensitive to the geometrical features of the porous nanostructures. Therefore, the key job when designing optimised nanostructures for enhancing nucleate boiling is to create optimal hybrid micro/nanostructures that can boost the generation of negative pressure, liquid supply towards and removal of vapor away from the nucleation sites.

Aggressive thermal management strategies such as liquid cooling have become essential for high-performance three-dimensional (3D) integrated circuit (IC) chips. Micro-pin fin arrays integrated between stacks can provide superior thermal performance with relatively less pumping power compared to microchannel cooling. In this work, we experimentally studied the single-phase heat transfer and pressure drop characteristics of micro-pin fin arrays. Three different samples consisting of 31–131 rows of cylindrical micro-pin fins with pin diameters D h = 45–100 μm, center-to-center pin spacings S = 74–298 μm, and pin height H f ∼ 200 μm were tested. Dielectric fluid R245fa was used as the working fluid with mass flow rates m = 14.7–181.6 g/min and corresponding Reynolds numbers Re = 35–481.3. The heat fluxes ranged from 2.5 W/cm 2 to 48.7 W/cm 2 , and the inlet fluid temperature was maintained at ambient temperature in the range of 22.2–25.3 °C. The local heater temperature distributions, average heat transfer characteristics, and pressure drops for various geometries of the embedded microfluid pin–fin arrays were determined. The experimentally determined heat transfer coefficient varied with both the mass flow rate and pin spacing with an averaged heat transfer coefficient of up to 18.2 kW/(m 2 ·K). Full-scale conjugate simulations with a turbulence model were conducted using ANSYS Fluent to validate the experimental results for the three cases. A comparison with the numerical model showed mean absolute errors of 9.1% for the heat transfer and 14.3% for the pressure drop.

Heat transfer enhancement and flow characteristics in grooved channel for pulsatile flow are investigated experimentally and numerically in the present work. The amplitude of the pulsatile flow is focused at different Reynolds numbers and oscillatory fractions by measuring time-varying flow rate through the electromagnetic flowmeter. In addition, the pulsatile flow patterns are visualized through aluminum dust method. The experimental results showed that the oscillatory fraction decreases with frequency of pulsatile flow, which is also proved by the flow visualization results. It is further shown that the amplitude of pulsatile flow can approach the setting value only when the frequency is lower than the critical frequency fc. It is found that the steady and unstable flow states exist in a pulsatile period. The unstable flow enhances fluid mixing between mainstream and recirculation vortex, and the flow mixing increases with the frequency increment, which is the main reason of heat transfer enhancement. Two-dimensional numerical simulations are further carried out to study this problem. The numerical results showed that heat transfer performance is better at high frequency. Moreover, the heat transfer efficiency decreases with oscillatory fraction when frequency is low. Phase shift is found to exist between the pulsatile flow rate, the outlet temperature and the area-averaged wall Nusselt number. Based on the experimental and numerical results, it is found that higher frequency and small oscillatory fraction could cause a better heat transfer performance in the grooved channel.