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This paper aims to investigate the hydrothermal and entropy generation characteristics of a non-Newtonian nanofluid containing CuO nanoparticles in an offset strip-fin microchannel heat sink (MCHS). The base fluid is solution of 0.5 wt% Carboxymethyl Cellulose (CMC) in water. This study investigates the effects of nanoparticles concentration, Reynolds number and geometric size of strip-fin on the performance of MCHS from the viewpoint of both the first and the second thermodynamic law. The results reveal that enhancing the Reynolds number improves the performance of MCHS by boosting the convective heat transfer coefficient of the working fluid which favourably reduces the CPU surface temperature and thermal entropy generation rate and importantly leads to the temperature uniformity of the CPU surface. However, increase in Reynolds number adversely affects both the pumping power and the frictional entropy generation in the system. Therefore, the optimal strip-fin size is investigated to find the optimum performance of the offset strip-fins MCHS from the viewpoint of both the first and the second thermodynamic law. The optimal results show that the highest ratio of heat transfer enhancement to pressure drop increment, using the nanofluid instead of base fluid, is 2.29. In addition in the optimal case, the minimum total entropy generation rate of the nanofluid is 2.7% less than the base fluid.

Subcooled flow boiling heat transfer is one of the most preferred heat removal methods, especially in areas where high heat fluxes are encountered. The design of heat sinks employing subcooled flow boiling involves prediction of pressure drop, in addition to heat transfer coefficient, as it exerts a huge influence on the local heat transfer, stability of the system and the critical heat flux. This paper provides a systematic review of the performance of the prior correlations and their applicability for different conditions based on a large experimental database compiled from 12 sources containing 1365 data points in total, focussing on water as the working fluid. The results of the assessment are used to identify the right correlation applicable for a specific operating range. New correlations are proposed for the cases and ranges for which the predictability of the prior correlations is unsatisfactory.

The thermo-hydraulic and entropy generation characteristics of a square substrate imposed to a uniform heat flux (q″ = 200 W/m2) have been numerically investigated in the laminar regime (99.6 ≤ Re ≤ 1998) using finite volume method of Ansys Fluent R16. Six new constructal designs triangular (Case-1), dumbbell (Case-2), diamond (Case-3), H-shaped (Case-4), two-lobbed (Case-5) and three-lobbed (Case-6)) are obtained when the embedded pipe is freely morphed to have a greater and greater access to the heated regions of the square plate. First, the convection effect is neglected and the temperature of the plate as a function of the length of the loop is quantified. Second, the convection is taken into consideration and the temperature of the plate for all such configurations is quantified. Also, the pressure drop and entropy generation for all the designs have been compared with each other to get the best design among them. It has been observed that the diamond shaped configuration provides better cooling to the plate when their lengths are in range of 2 ⩽ L c / L ⩽ 2.74 . For longer loops, H-shaped and three-lobbed designs are found to be the better design than the other designs. The pressure drop and total entropy generation for case-3 is found to be the lowest among all designs.

A facile chemical etching method for fabricating copper surface with hierarchical nanostructures and low surface energy coating was adopted in this study. Experimental investigation on the condensation heat transfer of refrigerant R141b in a rectangular microchannel with hydraulic diameter of 0.67 mm was conducted. The mass flux was of the range from 70 kg/(m2 s) to 850 kg/(m2 s). The effects of surface wettability and wall sub-cooled on the flow and heat transfer characteristics were analyzed. For channels with same nanostructures, the increase of contact angle resulted in the increase of heat transfer coefficient, but it had little effect on the pressure drop during condensation. The experimental results showed that the channel with the contact angle of 21.6° had a 12.81% enhancement in the heat transfer coefficient compared to the original copper channel with the contact angle of 12.8°. Moreover, a composite channel with gradient contact angle had a 16.67% enhancement in the heat transfer coefficient compared to the original one. The hierarchical nanostructures had significant influences on the heat transfer characteristics and pressure drop during convective condensation in microchannel. A higher mass flux resulted in higher heat transfer coefficient and pressure drop, while a higher wall sub-cooled temperature caused a lower heat transfer coefficient during the R141b condensation in a microchannel.

The present work investigates the heat transfer between the immersed heater and bed and hydrodynamics of the bed in swirling fluidized bed. The heat transfer process and bed hydrodynamics characteristics for swirling fluidized bed reactor (SFBR) were compared to those of conventional fluidized bed reactor (CFBR). Then the influence of particle diameter on the bed flow characteristics and heat transfer process in SFBR were studied. The heat transfer coefficient was calculated at different superficial air velocities, radial positions, and bed heights. Compared to a CFBR, the bed pressure drop and distributor pressure drop for SFBR were lower, that characterizes swirling bed reactor. The heat transfer coefficient of SFBR was higher than the corresponding of CFBR at lower portion of the bed and it was lower at the upper portion of the bed for all radial positions and for all air superficial velocities. Increasing particle diameter in SFBR resulted in decreasing both the heat transfer coefficient and bed pressure drop.

In this work, the potentiality of R513A and R1234yf as replacement refrigerants for R134a is experimental and numerically studied. The former substances exhibit Global Warming Potentials (GWP) that are 44% and 0.3% of the value for R134a (GWP = 1300). Thus, due to these lower values of GWP, they are promoted to substitute R134a according to environmental considerations. Nevertheless, these considerations should take into account the global environmental impact of the system, which depends not only of GWP but also of the energy consumption of the compressor. This last parameter depends on the thermal performances of the refrigerant. The research focuses on the performances of the diabatic phase-change processes within a mini-channel tube at high condensing temperatures. The use of this geometry is because mini-channel heat exchangers allow decreasing refrigerant charge and, consequently, direct emissions. Indirect emissions are also reduced because of the high heat transfer coefficients (HTC) obtained at these geometries. The results, based on experimental data and empirical correlations to fit them, show, for the tested experimental conditions, that R134a presents higher frictional pressure drop and HTC than R513A and R1234yf. Therefore, the substitution of R134a by the studied alternatives is analysed in detail to determine the power consumption in the compressor, the charge of the system, and the global heat power transferred. The analysis includes an estimate of the Total Equivalent Warming Impact (TEWI) in several scenarios to determine under which conditions, the substitution of R134a by R513A or R1234yf is a better alternative.

Shell and tube heat exchangers are the most important equipment in chemical processes plants. The heat exchanger design from the thermodynamic point of view is based on two fundamental aspects, the global heat transfer coefficient and the pressure drop. In 2007 a new thermodynamic property denominated “Entransy” was defined, which expresses the capacity of a body to transfer heat. This property can totally characterize the capacity to transfer heat of a shell and tube heat exchanger. The loss of this capacity is denominated “Entransy Dissipation”. For evaluating the ecological impact of thermal machines, Angulo-Brown created in 1991 the “ecological function”. In this paper the “entransy dissipation” and the ecological function were combined and a new expression for evaluating the ecological impact of shell and tube heat exchangers is proposed. A multi-objective optimization of shell and tube heat exchangers considering irreversibility is also carried out. The ecological function and the cost were used as objective functions. For solving the of multi-objective optimization problem, the method of the Genetic Algorithms is used.

In this paper, a realistic mathematical model is presented to study the performance of photovoltaic thermal system (PVT). The model considers the effect of control parameters like shape factor ( Z ) and mass flow rate ( m ) simultaneously to optimize the efficiency of PVT system. Simulation process based on iterative method is applied to different types of absorber plate shapes namely v-groove and curved shape grooves. To the best of our knowledge curved groove absorber plates has not been considered in the past. Photovoltaic and outlet temperature, electrical and thermal energy and exergy efficiencies, pressure drop are evaluated as a function of mass flow rate using Nusselt and Reynolds number for different Z values. The results indicate fall in temperature both for PV and outlet temperature and rise in energy and exergy values and pressure drop with mass flow rate. It is predicted that efficiency of PVT system can be improved when the value of Z lies in the range ( 1.3 ≤ Z 2 ) which corresponds to curve shape grooves in absorber plate. Further it is found that curved shaped grooves in the absorber plate can be a good substitute to V-grooves for low and high values of m .

To predict the slurry temperature in wet flue gas desulfurization system, a theoretical model was put forward and the calculation method was set up, the correlation between the parameters of inlet flue gas and the slurry temperature was first found. Based on the predicted slurry temperature, a 1-D coupled model of droplets motion, heat and mass transfer and pressureloss of liquid-gas two phases was established, and the numerical solutions were acquired by Runge-Kutta method, meanwhile, field tests were made to validate its reliability. Additionally, the distribution characteristics of droplet velocity, temperature and relative humidity of flue gas, and pressure loss over the height of spraying zone were discussed. The effect of drop diameter and other factors on the performance of spraying process were analyzed. The results show that the slurry temperature and pressure loss agree well with the operation values with errors of less than 5% and 7%. Outlet flue gas is near saturation state under discussed range of parameters. The drop diameter is the main factor that affects the performance of wet flue gas desulfurization system.

A numerical study has been performed to investigate the flow and heat transfer characteristics of fluid flow through heat exchanger tubes fitted with perforated conical rings. The holes are circular, and the number of holes N is ranged from 0 to 10. The influences of perforated conical ring diameter ratios D 2 / D 1 = 0.4 , 0.5 and 0.6 and the hole diameter ratios d / D = 0.06 , 0.1 and 0.14 on average Nusselt number, friction factor and thermal performance factor are reported. This analysis is performed in the turbulent flow regime 4000 ⩽ R e ⩽ 14 , 000 and the governing equations are solved by using (RNG) k - ∊ model. Due to strong turbulent intensity, perforated conical rings lead to more flow perturbation and fluid mixing between walls and the core region, which has a significant effect on heat transfer enhancement. The recirculating flow through the holes can also improve the heat transfer and reduce the pressure drop through the heat exchanger tube. The results show that the Nusselt number is reduced up to 35.48% by increasing the number of holes from 4 to 10. The maximum thermal performance factor of 1.241 is obtained for the case of N = 10 , d / D = 0.1 and D 2 / D 1 = 0.6 at Reynolds number of 4000.

Recently, various configurations especially single U-tube ground heat exchangers have been studied to improve the thermal performance of ground heat exchangers. Although helical ground heat exchangers generally have better thermal performance than the others, only two simple types of them, a helix with an internal outlet pipe and helix with an external outlet pipe have been studied in past years. So, the novelty of this paper is comparing eight new types of helical ground heat exchangers to a single U-tube heat exchanger in terms of heat exchange rate, pressure drop, thermal resistance, effectiveness, and thermal performance capability, in order to find the best one. The considered models have the same heat transfer area, and all numerical simulations are performed by commercial CFD software. The validation of the numerical simulations is done based on experimental results. The results indicated that the triple helix heat exchanger has considerably the best thermal performance compared with other ground heat exchangers. It followed by double helix and helix (W-tube) ground heat exchangers those which approximately have the same thermal performance. In general, it can be concluded that a single U-tube heat exchanger has the worst thermal performance and the minimum pressure drop in comparison with all helical ground heat exchangers.

By using exhaust mode ventilated windows in buildings cooled by evaporative cooler, the air flow channel of the window can play the role of natural relief air path. Thereby, the cooling potential of the relief air can be recovered and used to improve the thermal performance of the ventilated window. This paper investigates the thermal performance of an exhausting ventilated window in a typical evaporatively-cooled space. In this study, a zonal model has been used for simulation. The numerical approach was validated with reported experimental data and good agreement was shown. Results showed that the heat gain and the maximum daily temperature of the interior glass pane of the proposed ventilated window are less than those of the conventional double-glazed window. Providing natural relief through ventilated window can cause extra pressure drop and consequently reducing cooling capacity. This may also disturb the occupant's comfort due to noise issue. Hence, the impacts of window aspect ratio and thickness of the air flow channel have been investigated as effective factors on pressure drop and sound power level. Eventually, a step by step procedure has been proposed for determining the geometric parameters of the ventilated window for practical use in evaporatively-cooled buildings.

A 3D CFD simulation has been conducted in order to investigate the thermodynamic and flow characteristics of a Vuilleumier heat pump. Vuilleumier heat pumps are machines with few components which need a heat source of high temperature to operate and in some cases additionally some work input. The simulated machine comprises of one cylinder inside which two displacers reciprocate. The design of the machine resembles two opposing GPU-3 Stirling engines which is an existing engine and it was based on calculations conducted with the use of a 1D analytical mathematical model. The CFD simulation run for several cycles until convergence was established and yielded the spatial and temporal distribution of the temperature, pressure and velocity inside the several spaces of the heat pump. The pressure drop along the components of the heat pump was calculated and also the flow reversal time instances at the interfaces where adjacent components coincide. Other mathematical models used in the past couldn’t provide information neither about the flow direction everywhere inside the machine nor about the opposing flow streams inside the common compression space. Moreover, the CFD model provided the heat quantities entering or exiting the heat pump. The two heat exchangers that are placed in order to absorb heat, resulted to reject heat for a small time period during the cycle from a section of their body. Furthermore, the heat flow did not follow strictly the temperature difference between the bulk gas and the solid wall. It was driven by the temperature difference between the gas near the wall and wall. This discrepancy indicates a counter flow inside the tubes of the heat exchangers. Finally, it was expected that the CFD machine would require an amount of work to operate, because it was designed based on an analytical model without losses. So, the CFD simulation, which includes pressure drop losses, resulted to require additional external work, which can be minimized designing a thick cold displacer rod.

Room-temperature magnetic refrigeration is a recently developed, environment-friendly refrigeration technology. A rotary-type magnetic refrigerator has been built with a modified double Halbach permanent magnet array, which can provide an average magnetic field varying from 0.04 T to 1.50 T at a length of 100 mm. To evaluate the performance of AMRs with geometries, active magnetic regenerators (AMR) filled with gadolinium plates, spheres and flakes were respectively tested and compared. System performance such as temperature span, cooling power and pressure drop were investigated at different utilization factors and operating frequencies. The results revealed three AMRs performed quite differently. With larger specific heat transfer areas and smaller hydraulic diameters, AMRs filled with flakes/spheres provided better cooling performance. A maximum no-load temperature span of 14.8 K was obtained by flake-AMR with largest specific heat transfer areas and least filled mass. Sphere-AMR achieved a maximum temperature span of 10.8 K at a cooling power of 10 W among AMRs. With the same porosity and filled mass, sphere-AMR performed much better than plate-AMR, although the latter had a smaller pressure drop and associated pumping work.

The compact heat exchanger is significant for hybrid and electric/fuel cell vehicles with high energy utilization efficiency. This contribution introduces the heat current method to propose a new solution for the design and optimization of heat exchanger structure by combining the empirical correlations of heat transfer and flow resistance. On the premise of decreasing the air flow length of a plate-fin heat exchanger, the tree traversal method is utilized to optimize the offset strip fins on both the hot and the cold fluid sides of a plate-fin cross-flow heat exchanger. Moreover, an experimental setup is built for testing the thermal-hydraulic performances of the original and improved heat exchangers. The results show that the difference between the experimental and the theoretical results is less than 3.23% which represents the heat current method is feasible for heat exchanger analysis and optimization. Meanwhile, the total heat transfer coefficient increases by 7.43% and the pressure loss on the air side reduces by 29.7% for the improved heat exchanger, which is more suitable and high-efficient for the limited space in a high-power vehicle. Finally, the corresponding functions of the total heat transfer coefficient and pressure loss with different air mass flow rates are constructed for future practical applications of the improved heat exchanger.

Dead-ended anode Polymer Electrolyte Membrane Fuel Cell (PEMFC) is promising in small-scale power generation applications due to its simple structure, high reliability, and low price. However, it necessitates the anode purge to remove the impurities accumulated in the anode channel, making the system operation suffer from the sudden anode pressure drop during the purge. In most cases, a widely used solution of many fuel cell manufacturers is to open up the anode inlet valve regularly to recover the fuel cell terminal voltage. Although simple, the performance would degrade in case of the operation condition change. To this end, this paper proposes a modified Iterative Learning Control (ILC) strategy, which can repeatedly improve the control performance by learning from the previous actions. A mathematical model is developed to analyse the variations of the voltage and the anode pressure, by taking into account the membrane water transfer and the nitrogen permeation. The purge action is triggered when the single-cell terminal voltage drops by 0.1 V. The dynamics of the anode pressure is investigated under different openings of the purge valve. The simulation results have demonstrated the efficacy of the modified ILC strategy for anode pressure regulation in mitigating the periodic purge influence and the model uncertainty.

This study proposes a novel design of an atmospheric gas burner that introduces the concept of heat recirculation combustion by means of porous media combustion technology. A combustor and heat recirculation unit, used for preheating the combustion air, are part of the assembly of the porous material. The combustor is a self-aspirating annular porous medium burner (APMB) formed by a packed bed of alumina spheres with a diameter of 10 mm. A porous radiant recirculation heater was integrated into the APMB to obtain an elevated air temperature. To stabilize flames at the middle of the porous media packed bed, the flow velocity of the mixture has to be equal to the flame propagation velocity, making this a critical criterion for the design of the burner. A critical variable required to determine the flame velocity is the primary equivalence ratio. It was predicted using the empirical formula derived from the experimental data, and considers the momentum equation and pressure drop in the porous medium burner. Operation of the burner with firing rates from 21 to 44 kW resulted in evaluated air temperatures of 136–252 °C, yielding maximal temperatures greater than the adiabatic flame temperature. The NOx emissions were low because of the combustion within a porous medium. The CO emissions were high compared with the conventional burner but still conformed to the requirements of the national standard.

Recently, thermoelectric generators (TEGs) used to recover waste heat from engines have been widely studied due to their promising advantages, such as free heat sources, no pollution, and silent operation. In this study, a converging TEG has been proposed based on typical plate-type TEGs, where the hot-side wall of the heat exchanger was designed with an inward incline. To investigate the influence of the tilt angle, a mathematical model was developed. Combining the numerical simulation results, analyses of the TEG performance and model accuracy were carried out under 5 cases of boundary conditions. The results show that (i) the design of the tilt angle can apparently improve the Reynold’s number and the convective heat transfer coefficient of the exhaust gas. (ii) The hot-side temperature of the thermoelectric module also increases, and the larger the tilt angle is, the greater the hot-side temperature becomes. (iii) Through a comparison of the results between the mathematical model and simulation, the model presents good agreement, and the maximum error of the hot-side temperature is 4.24%. (iv) Although the TEG power is apparently improved, the pressure drop has a substantial influence on the TEG net power. It seems that the optimum angle is α = 2 ∘ when the mass flow rate of the exhaust gas is greater than 37.25 g/s, which increases the TEG net power by 5.96% in case 1. The findings of this work may provide a guide to the design, theoretical analysis, and optimization of thermoelectric generators.

To improve the overall heat transfer of supercritical carbon dioxide (SCO 2 ) in a horizontal circular tube, this study proposes a novel kind of tube with cylindrically concaves and fillets on the circular surface. Several tubes are designed accordingly to investigate various parameters like θ (sectorial angle of concaves), r 2 (radius of concaves), r 3 (radius of fillets) and n (number of concaves) to observe their effects on the thermal performance. The heat transfer and pressure drop characteristics of SCO 2 in these tubes are conducted by applying a verified turbulence model at an operation pressure of 8.0 MPa, an inlet temperature of 323.15 K, a mass flow rate of 0.005652 kg/s and a heat power rate of 565.488 W. It is found that the heat transfer coefficient of all the designed cases are improved due to the secondary flow and the maximum value of the h/h 4 /(f/f 4 ) is 1.134 compared with that of the standard case 4, while the friction factors are not increased but in fact even lower without raising the vortex pairs. Finally, the heat transfer coefficient at the vicinity of the pseudo-critical point is studied and it is noted that the heat transfer is also enhanced at this region.

A key challenge concerning the design of mini-channel heat sinks pertains to minimization of flow maldistribution in parallel channels. Although this challenge has spurred several research efforts towards exercising control over flow resistance within parallel channels to enhance flow uniformity, some practical issues associated with fabrication and implementation, as well as the possibility of a high pressure drop render the efforts less useful. The present study concerns development of a strategy that utilizes staggered pin-fin arrays in the inlet header of mini-channel heat sinks with the aim of achieving uniform flow distribution within parallel channels without increasing total pressure drop. Two different header shapes, trapezoidal and rectangular, have been considered, and ten different pin-fin arrangements for each type of header have been designed to numerically investigate flow uniformity within channels. The three-dimensional computational domain included the entire heat sink consisting of inlet/outlet connection tubes, inlet/outlet headers, and twenty parallel rectangular mini-channels—each measuring 3 mm in hydraulic diameter and equidistantly spaced with a 2-mm solid-wall spacing. It has been demonstrated that compared to baseline inlet headers, best pin-fin arrangements obtained for both trapezoidal and rectangular inlet headers yield significant improvement in flow uniformity within channels along with uniform temperature distribution over the heat sink base with minimal influence on the pumping power requirement.

In this work, a novel pressure-driven heating system with a Ranque–Hilsch vortex tube and ejector to recover pressure losses and absorb energy from the ambient environment to heat pipeline natural gas (PNG) is proposed to take the place of the conventional heating system with a boiler. A vortex tube to produce the high-temperature and low-temperature streams simultaneously with its energy separation effect and an ejector to drive the internal subcycle within the system to heat the low-temperature stream by the ambient air are installed. They regulate the PNG pressure and absorb heat from the ambient air while maintaining simplicity and reliability. To study the performance of this new system, an analogical experimental rig was established with pressurised air as the working fluid. The experimental results verified the heating capacity of the system, in which a significant temperature rise greater than 9 K was obtained with a small pressure drop of approximately 1.0 MPa. Compared with a sole throttling valve, an energy-saving ratio of no less than 0.35 was achieved.

Due to growing environmental concerns about global warming, eco-friendly HFO refrigerants such as R1234yf have been recently introduced as an alternative to conventional HFC ones. The present research work investigates the boiling heat transfer coefficient and pressure drop of R1234yf flow as a replacement refrigerant for R134a in horizontal plain and wire-coil inserted tubes of 8.7 mm inside diameter. Meanwhile, the accuracy of some well-known correlations to predict R1234yf flow boiling thermo-hydraulic properties are evaluated. The results show that the studied correlations may predict the heat transfer coefficient and pressure drop of R1234yf flow boiling in plain tubes with an acceptable accuracy. The effect of main operating parameters including the vapor quality, the heat flux, and the mass velocity is also thoroughly studied using a well-instrumented test apparatus. According to the results, at constant vapor quality, the heat transfer coefficient increases with both the heat flux and the mass velocity. In addition, inserting a wire-coil in the tube may enhance the heat transfer coefficient as much as 72% while leads up to 85% increment in the pressure drop. This study and its result is helpful in design of next generation of heat exchangers use R1234yf refrigerant as a working fluid.

Ceramic materials enable operation in high temperature and harsh environments and can so exceed the capabilities of comparable metal heat exchangers. For the use in power plant applications and high temperature processes a heat exchanger has to operate under both high temperature and high pressure levels. So far, the operation of a ceramic heat exchanger under such conditions has not been investigated. This work presents a ceramic heat exchanger prototype with modified offset-strip-fin design, which was experimentally investigated in a specifically designed test environment. Experimental tests were conducted with air as a heat transfer fluid at temperatures up to 800 °C, absolute pressures up to 5 bar and norm volume flows up to 160 N m3 h−1. The functionality under high temperatures and absolute pressures could be proven. The paper reports measured performances of the heat exchanger prototype, for which heat transferred up to six kilowatt and effectiveness up to 97% has been achieved. A comparison of Colburn and friction factors with correlations from literature was conducted, too. Good agreement for Colburn prediction of the flue gas channels could be shown, but the investigation also reveals a limited applicability for the friction factor and the Colburn factor of the process gas channel.

A numerical analysis is conducted on flow and heat transfer of different configurations of miniature chamber with and without the rectangular pin as complex and smooth models, respectively. Five configurations of the miniature chamber are considered, and they are named for short as smooth models, including RC, HC1, TC1, HC2, and TC2, and complex models, including RC-RP, HC1-RP, TC1-RP, HC2-RP, and TC2-RP. Experiments are also carried out on a real straight case in order to validate the numerical results and create a baseline for the comparison. The working fluid is water in the laminar flow regime. It is found that the numerical results are in a good agreement with the current experimental data, whose the maximum differences are 5.3% for the heat transfer coefficient and 6.4% for the pressure drop. Depending on the studied case and Reynolds number (400

A water-cooled multi-die heat sink with parallel rectangular micro-channels was designed to satisfy the operational requirements of a multi-die processor. A shape optimization strategy based on the RSM (response surface method) was used to minimize pressure drop and die maximum case temperatures. The effects of the thermal interface materials and heat spreader between the dies and heat sink were captured by the numerical simulation. The optimization was performed for constant values of coolant flow rate and inlet temperature, as well as the power, location, and surface area of the dies. The influence of channel hydraulic diameter, Reynolds number, thermal entrance length, and total heat transfer surface area on the hydraulic and thermal performance of the heat sink was determined using CFD (computational fluid dynamics) simulations at RSM design points. A sensitivity analysis was performed to evaluate the effect of the design parameters on the response parameters. The optimum designs were achieved by minimizing a weighted objective function defined based on response parameters using JAYA algorithm. The results of weighted sum method were compared with Pareto based three-objective optimization with a NSGA-II (non-dominated sorting GENETIC algorithm). Finally, a parametric study was performed to see the effect of the design parameters on the response parameters.

This work presents a numerical investigation of convective heat transfer and confined ferrofluid jet flow through a channel under the influence of six magnetic sources placed outside the system and arranged in a staggered manner. The upper and the lower walls are maintained at a constant heat flux density, while the side walls at the inlet are thermally insulated. For the ferrofluid flow, the ferrohydrodynamics (FHD) and thermophoresis effects, as well as the Brownian motion are taken into account. The governing equations are solved numerically using the finite volume method with the SIMPLE algorithm. The influence of the magnetic number (Mn), the sources position (Xi), and the jet inlet height (R) on the flow as well as on the heat transfer characteristics is analyzed. The results show that the flow structure and the thermal boundary layers development are strongly disturbed by the presence of the magnetic sources as well as the narrowing at the channel inlet. The heat transfer is enhanced with the increase of the magnetic number Mn and the reduction of the opening ratio R. It is also found that the closer the sources from the channel inlet, the higher the heat exchange, with an optimal position depending on Mn and R. Finally, the comparison between the gain in heat transfer rate and the pressure drop indicates that the case R = 1/4 and Mn = 50 leads to the most efficient system.

Pin fin-interconnected reentrant microchannels (PFIRM) are proposed and developed for microchannel heat sinks cooling systems to combine both merits of reentrant cavities and micro pin fins in heat transfer enhancement. They feature two directions of reentrant microchannels with a intersection angle of 30° in the downside, and staggered arrays of micro diamond pin fins on the upper side. Micro-milling process is utilized to process the interconnected reentrant microchannels, which simultaneously induces the formation of micro diamond pin fins. Flow boiling experiments are conducted using two coolants (deionized water and ethanol) at inlet subcoolings of 40 °C and 10 °C, mass fluxes of 125–300 kg/(m2 s). Flow boiling heat transfer, pressure drop and two-phase flow instabilities of the PFIRM are systematically assessed. It is found that the PFIRM showed a 39–284% enhancement in two-phase heat transfer in water tests, and a 29–220% in ethanol tests compared to the parallel reentrant microchannels. Decrease of bubble confinement effect by the interconnected microchannels and ideal spaces for the bubble nucleation and movement supplied by the reentrant chambers contributed to the above boiling heat transfer enhancement of PFIRM. The PFIRM heat sink operated more efficiently using the coolant of water than ethanol.

This work is related to numerical investigation and optimization of the attack angle of delta winglet type vortex generators in plain and wavy finned-tube heat exchangers. For this purpose, three-dimensional numerical simulation data and a multi-objective genetic algorithm with Artificial neural network are employed. The heat exchanger models considered in this study are three rows of tubes in staggered arrangement. The angle of attack (α) of delta winglets and Reynolds number varies from 15° to 75° and 500 to 1300, respectively. Results are illustrated by investigating the flow structures and local heat transfer coefficient. Results show that compared to plain fins, wavy fins enhance the heat transfer performance of the heat exchanger due to constant disruption of thermal boundary. Applying delta winglets on the plain and wavy fins further increased the heat transfer by energizing flow in the wake of tubes while friction factor is also increased. To achieve a maximum heat transfer enhancement and a minimum pressure drop, the optimal angle of attack for varying Reynolds numbers are calculated using the Pareto optimal strategy. For this purpose, CFD data, artificial neural network and a multi-objective genetic algorithm are combined. Results show that compared to the corresponding smooth fins, heat transfer per unit volume (Qv) and pumping power per unit volume (Pv) performances of delta winglets in wavy finned-tube heat exchanger has increased up to 12.5% and 7.41%, respectively. Similarly, for plain finned-tube heat exchangers, the maximum increase in Qv is 14.7% while Pv is increased by 6.93%.

Recently, the use of air injection into heat exchangers to increase their thermal performance has attracted great attention. In the present study, air is injected into the coil side of a horizontal shell and coiled tube heat exchanger. An external unit for air bubbles injection is used. By means of this method, there is no need to modify the original design of the heat exchanger. The effect of various air volume fractions (VF) on the number of thermal units (NTU) and pressure drop is examined. Besides, the effect of air bubbles injection is analyzed from the viewpoints of the second law of thermodynamics, which has not been performed previously. The entropy generation, augmentation entropy generation number and Witte-Shamsundar efficiency as important criteria are analyzed for the developed system. The results revealed that air bubbles injection into the coil side improved the thermal characteristics of the heat exchanger. Also it was observed that for achieving the best performance the volume fraction should be optimized. Besides, according to the second law analysis, it is realized that air bubbles injection into the coil side is more effective method for the heat exchangers with high liquid flow rates.

Compact heat exchangers are used in the industry to perform the heat transfer between two fluids at different temperatures. Characterized by high heat transfer area and rate per unit volume, they are mainly used for applications where space is restricted. In the current study, a complete analysis of heat transfer and pressure drop of a diffusion bonded stainless steel heat exchanger is performed. A model to predict the pressure drop in the heat exchanger is presented, highlighting the contribution of the major components of the equipment: core and headers. Three correlations, which main input parameter is the dimensionless hydrodynamic length, were used to predict the friction factor inside the channel. A cross-flow heat exchanger with mini square cross section channels, of 3 mm of edge, was manufactured. In order to validate the model and to study the thermal and hydrodynamic behavior of the equipment, an experimental test apparatus was developed. The heat exchanger was tested with water at different levels of mass flows. Experimental thermal results were compared with the theoretical model with small disagreement. To improve this comparison, an adaptation of a Nusselt number correlation, for square channels, was proposed, showing now a good convergence to the data. The hydrodynamic results showed that headers were responsible for at least 50% of the heat exchanger pressure drop. Besides this study, numerical simulations were performed. Finally, numerical, experimental and theoretical results were compared, showing similar trends, validating, this way, the theoretical model and the numerical procedures.

A three-dimensional numerical model has been conducted to study the laminar flow and heat transfer characteristics in serpentine microchannel with different geometric fan-shaped reentrant cavities for Reynolds number ranging from 150 to 980. The microchannel has the width of 0.1 mm and depth of 0.2 mm in the rectangular cross-section. The geometric parameters of fan-shaped reentrant cavities examined in this paper are 0.04058 mm for the width (Wc), 0.0084–0.02029 mm for the height (Hc) and 0.04058–0.12174 mm for the spacing (Sc). This study firstly presents the comparison of the thermal and hydraulic characteristics of serpentine microchannel with fan-shaped reentrant cavities with the conventional microchannels heat sink. Then the effects of fan-shaped cavities on velocity contour, temperature distribution and secondary flow features in such microchannel heat sinks are given. It is apparent that the fan-shaped cavities can effectively reduce the flow resistance and improve the temperature distribution uniformity. The effects of developed non-dimensional variables on the frictional pressure drop, average Nusselt number and the relationship between thermal resistance and pumping power in the novel channel are investigated. Compared with rectangular serpentine channel, heat transfer is enhanced for all cases and flow resistance is decreased besides Re = 980 and Hc/Wc = 0.2887. The best performance of serpentine microchannel with fan-shaped reentrant cavities is found at Sc/Wc = 3 and Hc/Wc = 0.2887.

To provide a favorable temperature for a Li-ion battery liquid cooling system, novel leaf-like channels with loops were designed as the dispersed layer. Furthermore, four collection channels along diagonal lines were established to decrease the temperature gradient, which formed a double-layer cooling channel. The effects of four structural parameters (width ratio, length ratio, bifurcation angle and channel thickness) on maximum temperature and surface standard deviation were investigated. In addition, the influence of the inlet mass flow rate was also discussed. The obtained results demonstrated that optimal maximum temperature and temperature uniformity were achieved at a width ratio of about 3/4, a length ratio close to 0.5 and a bifurcation angle in the range of 30–50°. Moreover, the thickness of the cooling channel had a strong effect on the pressure drop but only a little on the heat transfer. Increasing inlet mass flow rate can obtain the required heat dissipation capacity but at the expense of pressure drop. In summary, the synergistic effect of thickness and inlet mass flow rate can satisfy the requirements of both heat dissipation and power consumption. This optimized design can decrease the maximum temperature from 60 °C to 32 °C and the standard deviation of surface temperature from 7.1 °C to 1.4 °C. These simulation results are helpful for the design of cold plates with fractal networks.

The use of an air-cooled micro-channel heat sink is becoming increasingly widespread to cool supercomputers. Passing air through micro-channels, which acts as an absorber, is the main heat transfer mechanism for cooling the main board of supercomputers. The present study aims to investigate the 3D flow of air on the performance of micro-channel heat sinks in which the channels can have five different configurations (e.g., circular, hexagonal, rectangular, triangular and a straight slot). The effect of the flow rate (Reynolds number) and inlet temperature of air on the Nusselt number, heat transfer coefficient, pressure drop and required pumping power were investigated. The results showed that among the five channel configurations, the triangular shape provides the highest thermal performance for cooling a specific micro-channel heat sink. However, with respect to manufacturing cost, the straight slot configuration is recommended. In the next stage, the effect of the manufacturing material was considered with respect to aluminum, alumina (92%), cobalt, stainless steel, copper, and silver thermal conductivity. In the final stage, the effect of the air flow rate on the thermal stress and deformation of the geometry were studied, which provided that the coolant flow rate from 60 to 120 CFM prevents body failure. The results of this study could be helpful in the design of the cooling system of supercomputers.

The current paper attempts to numerically examine the hydrothermal characteristics and energy performance of a hybrid nanofluid containing graphene nanoplatelet–platinum composite powder in a triple-tube heat exchanger equipped with inserted ribs. The nanofluid flows in the inner annulus side, whereas the cold water and normal water flow in the tube side and outer annulus side, respectively. The ribs are installed on the outer surface of the inner tube. The overall heat transfer coefficient, effectiveness and heat transfer rate of the heat exchanger are enhanced by increasing the nanoparticle concentration and rib height and by decreasing the rib pitch. The pressure drop is more intense in the cases of smaller rib pitch and higher rib height because the particles pass longer routes under these conditions. Based on the heat transfer enhancement, the case with greater rib height and smaller rib pitch at the highest concentration is suggested, because demonstrates the greatest thermal performance for the heat exchanger. Whereas, in the view of the energy efficiency, the heat exchanger with the smaller rib height and lower rib pitch with the highest nanoparticle concentration is recommended since the performance index of this case is greater than that of the other cases.

Heat transfer augmentation by using vortex generator (VG) as an enhancement method in plain fin-and-tube heat exchanger (FTHE) has recently captured high attention of many researchers. In the present study, the flow characteristics and thermal performance across the FTHE with and without trapezoidal winglet vortex generator (TWVG) are investigated using dye injection technique by experimental approach and numerical simulation using finite volume method. Two types of TWVG are used, namely flat trapezoidal winglet (FTW) and curved trapezoidal winglet (CTW), either arranged in common flow up (CFU) or in common flow down (CFD) arrangements. In this study, Reynolds number (Re), angle of attack (β) and aspect ratio (Ʌ) in the range of 500–2500, 10–90° and 2–3 were used respectively. From the experimental study, the results show that the heat transfer can be enhanced by using certain TWVG geometry, arrangement, Ʌ and β to reduce the wake region size at the rear portion of the tubes. Meanwhile, from the numerical simulation, augmentation towards the heat transfer rate is normally accompanied with higher pressure drop penalty. It is inferred that FTW in CFU arrangement at Λ = 3 and β = 10° was found to give the best thermal and hydraulic performance across the FTHE channel when both heat transfer enhancement and pressure drop penalty are considered.

The objective of this work is the investigation of a nanofluid-based compound parabolic collector under laminar flow conditions. The examined nanofluid is Syltherm 800/CuO with 5% volumetric nanoparticle concentration and the examined temperature range is from 25 °C up to 300 °C. The analysis is performed in SolidWorks Flow Simulation Studio using a developed computational fluid dynamics model. Moreover, it is essential to state that this work is conducted under laminar flow conditions because the laminar flow regime is common for the compound parabolic collectors. It is found that the mean value of the thermal efficiency is 1.24% while the maximum 2.76% for inlet temperature equal to 300 °C. Moreover, it is found that the mean and the maximum heat transfer coefficient enhancements were 16.16% and 17.41% respectively, while the maximum pumping work increase was found to be 16.09%. However, the calculated pumping work demand was too low in all cases and this fact indicates that the increase of the pressure drop with the nanofluid is not a limitation in the present problem. In the end, the exergy efficiency enhancement is found to be up to 2.60%, while the overall efficiency enhancement up to 2.76% with the use of nanofluid.

An experimental study of steam condensation in a power-plant air-cooled condenser is presented. This is the third of a four-part group of papers. The first two parts (Kang et al., 2017; Davies et al., 2017), published in the same journal present the facility, pressure drop, void fraction, and flow regime results, while this study presents heat transfer results and analysis. A follow-up paper will investigate the effect of varying inclination angle. The condenser test section is half of a flattened steel tube with brazed aluminum fins. The full size of a condenser tube is 10.72 m × 214 mm × 18 mm. The condenser tube is cut in half lengthwise and covered with a polycarbonate window to perform visualization simultaneously with the heat transfer measurements. All tests are performed with condensing pressure slightly above atmospheric. Stratified flow is found for all test conditions and all locations along the condenser, with both filmwise and dropwise condensation along the condenser wall. Steam-side heat transfer coefficient is found to depend on wall-steam temperature difference, and not quality or Reynolds number for vapor. As a result, steam-side heat transfer coefficient does not decrease along the condenser length, as is common for smaller condenser tubes with higher mass flux. This phenomenon disagrees with the predictions of many of the published correlations. Overall condenser heat transfer coefficient is found to decrease along the condenser length, due to an increase in the thickness of the stratified condensate layer.

New theoretical and empirical relationships for the Nusselt number on the liquid-side in the transitional and turbulent range were derived, using various procedures for calculating the friction factor. The water-side friction factor in the transition flow range Re w , t r b ⩽ Re w ⩽ Re w , t r e was calculated by linear interpolation between the values of the friction factor for Re w , t r b = 2100 or Re w , t r b = 2300 and Re w , t r e . It was assumed that the transitional flow ends at Re w , t r e = 3000 . Other values of Re w , t r b and Re w , t r e determined experimentally can be adopted based on experimental data. Alternatively, the friction factor in the correlation for the Nusselt number on the liquid side was calculated using the Churchill formula valid for all fluid flow regimes in smooth and rough pipes. The paper presents new results of experimental testing of a new design two-pass car radiator with two rows of tubes. New empirical heat transfer relationships for the air and water-side were determined based on experimental data using the least squares method. Also, the empirical formulas for the water-side pressure loss coefficient and air-side friction factor were developed. The temperature of the water at the outlet of the heat exchanger was calculated using P-NTU (Effectiveness-Number of Transfer Units) method. The theoretical and empirical correlations for the water-side Nusselt number agree very well. By using the P-NTU method, the procedure proposed in the paper can be used to determine the heat transfer correlations in heat exchangers with different constructions.

Due to stringent emission standards, diesel particulate filter (DPF) is used to reduce and control particulate matter (PM) emissions for engine manufacturers. The periodical regeneration the DPF by oxidizing the accumulated PM is able to avoid pressure drop build-up and fuel efficiency decrease. This paper discusses the influence of regeneration temperature, regeneration flow rate and regeneration time on both regeneration and emission performance by regeneration test bench. From the temperature distribution profiles, the maximum temperatures were achieved at rear and center part of DPF in all the regeneration tests. When the regeneration temperature was higher than 525 °C, it was benefit to emit the large diameter particles. Increasing the flow rate had negative effect for the maximum temperature, maximum temperature gradient, and regeneration performance ratio. The trends for regeneration efficiency, total mass concentration and emitted particle average diameter were similar under different regeneration flow rates. Increasing regeneration time was benefit for improving regeneration efficiency, but it had negative effect for the performance ratio. The optimization active regeneration operating parameters were achieved after considering both regeneration and emission characteristics.

A crucial aspect of Joule-Thomson cryocooler analysis and optimization is the accurate estimation of frictional pressure drop. This paper presents a pressure drop model for boiling of non-azeotropic mixtures of nitrogen with hydrocarbons (e.g., methane, ethane, and propane) in microchannels. These refrigerant mixtures are important for their applicability in natural gas liquefaction plants. The pressure drop model is based on computational intelligence techniques, and its performance is evaluated with the mean relative error (mre), and compared with three correlations previously selected as most accurate: Awad and Muzychka; Sun and Mishima; and Cicchitti et al. Comparison between the proposed artificial neural network (ANN) model and the three correlations shows the advantages of the ANN to predict pressure drop for non-azeotropic mixtures. Existing correlations predict experimental data within mre = 23.9–25.3%, while the ANN has mre = 8.3%. Additional features of the ANN model include: (1) applicability to laminar, transitional and turbulent flow, and (2) demonstrated applicability to experiments not used in the training process. Therefore, the ANN model is recommended for predicting pressure drop due to accuracy and ease of applicability.

The single-phase thermal hydraulic characteristics of liquid metal sodium are very essential for the design and safety analysis of sodium-cooled fast reactor (SFR). In this paper, the pressure drop and heat transfer features of single-phase liquid sodium were experimentally investigated in a 7 rod bundle with the velocity range of 0–4 m/s, heat flux up to 120 kW/m2 and the absolute pressure range of 0–0.2 MPa. The corresponding Reynolds number ranges from 4000 to 40,000, and the Pe number varies from 0 to 340. It was found that the critical Re number for transition-turbulent flow of single-phase liquid sodium is 13,500 in the hexagonal 7-rod bundle. Then the effects of relative axial position, wall heat flux and Re number on the heat transfer were discussed, respectively. Some existing correlations in the literatures were assessed and compared with the experimental data. Results indicated that these correlations could not predict the current experiments well because of the different geometries and working fluids. The new correlations for the friction factor and Nu number calculations were proposed based on the current experimental data. For 98.5% of heat transfer data produced by the other researchers, the prediction error of the new correlation is less than 30%. For most of the experimental data, it is less than 20%, which sufficiently proves that the correlation developed in this paper could give a good prediction of the experimental data obtained by other researchers.

Drift eliminators play a major role in cooling tower operation. They are designed to reduce the discharge aerosols in their exhaust air stream to a minimum since inhaled airborne particles can cause the well-known Legionnaires disease. However, the pressure drop induced into the air stream increases the power consumption of the system. Accordingly, the design and selection of these elements should be a trade-off between the pressure drop and the collection efficiency. In this paper, six commercial drift eliminators (vane, wire mesh, and honeycomb-type) have been characterized in terms of pressure drop and collection efficiency with the aim of providing reliable information that can be used in drift eliminator design and selection. Fifty-three experiments were conducted regarding the pressure drop and collection efficiency characterization of the eliminators. Generally speaking, for the same type of eliminators the higher the pressure drop, the more efficient it is. Concerning typologies, wire-mesh eliminators perform better than the rest in terms of both pressure drop and collection efficiency. Dimensionless correlations for the pressure drop coefficient and the collection efficiency have been developed for the tested eliminators, showing a good agreement with the experimental results. A selection criterion has been proposed based on the dimensionless parameters that govern the problem and the experimental data of power consumption. It is based on determining the power consumed by the fans of the tower by setting a limit of collection efficiency and the droplet distribution characteristics at the cooling tower outlet area.

In the present work, an experimental investigation was conducted on the fluid flow and heat transfer characteristics of gallium-indium eutectic flowing in the microchannel passages. The overall heat transfer coefficient, pressure drop, friction factor and the thermo-hydraulic performance of the microchannel was experimentally quantified for various compositions of indium in the eutectic mixture. Influence of low-frequency vibration at two frequencies and amplitudes on the overall heat transfer coefficient and thermo-hydraulic performance of the microchannel was also studied and discussed. Results showed that the eutectic mixture has a higher thermal performance in comparison with gallium (78% enhancement in thermo-hydraulic performance). Also, the vibration increased the heat transfer coefficient of the microchannel such that at a frequency of 50 Hz and an amplitude of 10 m/s2, the highest heat transfer coefficient and thermal performance of the microchannel was registered. However, the effect of the frequency was more tangible for the eutectic mixtures with lower viscosities such as 95%Ga-5%In and pure gallium. Also, neither the frequency nor the amplitude changed the pressure drop value of the system. Interestingly, the presence of the indium in the eutectic mixture enhanced the heat transfer coefficient, which was attributed to the enhancement in the thermal conductivity of the eutectic mixture. The augmentation in the value of the pressure drop was also compensated with the anomalous increase in the heat transfer coefficient. This study revealed the plausible application of Ga-In mixture in high heat flux cooling systems inside the microchannel heat exchangers.

Numerical modelling can significantly contribute to the performance enhancement and technological advancement of Stirling engines. In this paper, a practically feasible thermodynamic model was developed for beta type of Stirling engines with rhombic-drive mechanism. Quasi-steady flow approach was adopted to analyze the heat transfer and flow friction effects of the heater, cooler, and regenerator on the performance of engine. The numerical model predicts the output power and thermal efficiency while considering the pressure drop in heat exchangers and numerous power and thermal losses. A parametric study is utilized to investigate the impact of operating and geometric parameters on the power output and efficiency of the Stirling engine. A combination of optimized temperature ratio, swept volume, regenerator matrix porosity, phase angle, pressure and engine frequency are assessed. The optimized model is then compared and validated against the experimental data obtained from the General Motor’s prototype of GPU-3 Stirling engine. Substantial improvement on the performance of the engine is achieved by optimizing the operating and geometric parameters of the engine.

An experimental study was carried out to investigate the heat transfer, pressure drop and flow instability characteristics associated with the flow pattern of deionized water during two-phase boiling in a silicon-based manifold microchannel heat sink coated with silicon nanowires (SiNWs) compared to a plain-wall device. The manifold microchannel device featured parallel transverse microchannels etched on a silicon substrate and longitudinal microchannels etched on a glass cover plate. Silicon nanowires were generated on the bottom and the sidewalls of the silicon microchannels. A closed-loop experimental system was constructed to demonstrate thermal and hydraulic performance. Experimental results were presented with mass fluxes ranging from 250 to 1250 kg/m2 s and subcooled inlet temperatures from 15 K to 65 K. Results for the SiNWs device showed an approximate 20% improvement in heat flux rejection compared to the plain-wall device under the same wall superheat conditions. A subcooled inlet temperature of 65 K associated with a mass flux of 1250 kg/m2 s is shown to be capable of dissipating an effective heat flux of 431.3 W/cm2 with a wall superheat of about 85 K. Overall, the SiNW coatings proved positive effects on enhancing the flow boiling heat transfer with slower pressure drop increase, meanwhile the three-dimensional manifold microchannel design is revealed to effectively mitigate flow instability during the entire single and two-phase flow regions. This indicates great potential in utilizing three-dimensional flows by integrating SiNWs surface structures in high heat flux cooling applications.

Twisted oval tube is also a finned tube in a sense, especially when used in crossflow, although the fins of the twisted oval tube are invisible. An experimental research on heat transfer and pressure drop performance of twisted oval tube bundle with staggered layout is conducted. The correlations of Nusselt number and pressure drop coefficient deduced from this paper are well compatible with experimental data, which could provide a theoretical reference of twisted oval tubes for industrial applications.

An experimental study of convective steam condensation inside a large, inclined, flattened tube used in air-cooled condensers for power plants is presented. This is the fourth of a four-part group of papers. The first three parts (Kang et al., 2017; Davies et al., 2017, 2018) published in the same journal present the facility, pressure drop, void fraction, flow regime and heat transfer results, while this study presents the effects of inclination on heat transfer and flow regimes. The condenser is a flattened steel tube with brazed aluminum fins. The full tube has dimensions 10.72 m × 214 mm × 18 mm. The condenser tube is cut in half lengthwise and covered with a polycarbonate window to perform visualization simultaneously with the heat transfer measurements. The steam is condensed at atmospheric pressure, and cooled by air at a uniform velocity profile. The angle of inclination is varied from horizontal (0°) to 75° downward. Condenser performance is also predicted with a model. The majority of the condenser is found to be in the stratified flow regime for all inclinations tested, with only a short annular section at the inlet of the condenser. The tubes inclined greater than 60° are also found to have stratified-wavy flow near the condenser outlet. Overall condenser U is found to increase with increasing downward inclination angle of the condenser, with a maximum increase of approximately 4% at 75° inclination. This improvement is found to be the result of improved drainage and increased void fraction near the condenser outlet. Mean steam-side heat transfer coefficient 1 (HTC) is found to remain constant along the tube, and for the entire condenser, with changes in tube inclination angle. Commonly-used inclined condensation HTC correlations are found to underpredict the magnitude of the experimentally-determined steam-side HTC.

Microchannel has been widely used in the microelectronics device because of the excellent heat transfer and low manufacturing cost. In this study, three kinds of microchannels including interlaced microchannel (IM), parallel microchannel (PM) and spiral microchannel (SM) were designed. The influences of microchannel configurations and hot/cold water parameters, such as flow rate and entrance temperature, on the thermal and hydraulic performances were studied. For IM, the heat could be transferred to the adjacent two cold water channels from the hot water channel, because the two side walls of the channel were served as the main heat transfer surfaces. The heat transfer area of IM was 6.4 and 8.4 times that of the PM and SM, respectively. The results showed that the heat transfer performance of IM was significantly enhanced without an obvious increase of pressure drop. The heat transfer performance was improved and the thermal resistance was reduced with the flow rate and entrance temperature of cold water. However, the flow rate and entrance temperature of hot water had little influence on the heat transfer performance. The IM exhibited an excellent thermal and hydraulic performance, and showed a good reliability and operation stability with a larger range of heat load.

High filtration for ultrafine particles (UFPs) has many potential energy-saving and environmental applications. In this study, riblet surfaces with different geometries were investigated for their enhanced UFPs deposition in duct flow conditions for the first time. Polydisperse sodium chloride (NaCl) particles as UFPs were used and tested in a wide range of Reynolds numbers, ranging from laminar to fully turbulent flow. The upstream and downstream size-resolved particle concentrations of the NaCl particles were measured. Experimental results show that deposition velocity increased with Reynolds numbers and decreased with particle size for both empty duct and duct furnished with riblet surfaces. Duct with riblets surface had the highest enhanced deposition velocity ratio for UFPs in the laminar regime, which could reach about 7.0 times for particles less than 20 nm, and about 4.8 times for particle size between 20 nm and 50 nm. The experimental results reveal that the friction factors of the duct with riblet surfaces were close to that of the empty duct in the turbulent flow, while higher pressure drop penalty was observed for riblet surfaces in the laminar flow. The performance index of riblet surfaces by combining enhanced UFPs deposition and high frictional drag was also evaluated. The results show that the highest performance ratio of riblets could be up to 4.0. In addition, the riblet surface with largest protrusion height generally give the best performance.

An experimental investigation concerning the convective heat transfer of n-decane maintained at supercritical operating pressures and flowing through a 2-mm-diameter and 200-mm-long pipe under conditions of varying rotational speeds, mass flow rates, inlet temperatures, and heat flux has been reported. The wall temperature was measured at four positions around the pipe periphery at five selected cross-sections along the pipe length. Maximum convective heat transfer was observed along the trailing edge of the centrifugal section while its corresponding minimum was observed along the leading edge. Heat transfers along the two sides of the channel were observed to be nearly identical. The average sectional convective heat-transfer coefficient demonstrated an increase with increase in rotational speed, and its value corresponding to 1500 rpm was observed to lie within the range of approximately 2–2.5 times its corresponding value under static conditions. The strong effect of the buoyancy force caused by the centrifugal force and flow deceleration due to pressure drop tended to limit the deterioration of local heat transfer. This study proposes an expression for a dimensionless criterion concerning the buoyancy force generated under rotating conditions along with a new local Nusselt number correlation apropos the heat transfer of n-decane flowing through a rotating centrifugal section under supercritical operating pressures.