Heat Transfer Enhancement Ratio Research Articles

Heat transfer and entropy generation characteristics of nanofluid flow over bluff bodies under steady and unsteady flow: A two-phase approach

Owing to higher thermal conductivity, nanofluids have the potential to be the coolant for various applications ranging from internal to external flows. A two-phase model is implemented to model the interaction between nanoparticles and base fluid to obtain accurate results. Heat transfer and entropy generation characteristics of nanofluid (Al2O3 and water) flow over bluff bodies such as circular and square cylinders for steady (20 < Re < 100) and unsteady (Re = 150 and 300) flow conditions have been carried out for various volume fractions (0.5–2 %). The same has been expressed in quantitative and qualitative aspects with parameters such as mean Nusselt number, surface Nusselt number, heat transfer enhancement ratio, and entropy generation. Heat transfer rate increases with an increase in flow rate and volume fraction for both steady and unsteady flow. Heat transfer enhancement in steady flow ranges from 1.10 to 1.35. For unsteady flow (Re = 150 & Re = 300), nanofluid's heat transfer enhancement ratio is higher than water in the range of 1.10–1.8. This is attributed to the early separation of flow and the presence of large recirculatory regions. With the increase in Re, the entropy generation decreases for circular and square cylinders. Compared to nanofluid, the entropy generation is higher for water.

Experimental study on the pool boiling heat transfer of R134a outside various enhanced tubes

The paper aims to experimentally study the pool boiling heat transfer properties of R134a outside various enhanced tubes under saturation temperatures ranged from 3 to 40 °C and heat fluxes of 5.5∼76.5 kW·m-2. One smooth tube and three micro-fin tubes were selected as the tested tube. Based on the experimental results, it can be found that the pool boiling heat transfer coefficient increases firstly and decreases later with increasing heat flux, but saturation temperature and water velocity have little effect on the heat transfer coefficient. Under the same experimental conditions, refrigerant thermal resistance ratio is greater than 0.5 during the heat transfer process. The micro-fin facilitates the bubble nucleation with the heat transfer enhancement ratio varying in the range of 2.08∼4.59. Moreover, the experimental values are compared with the calculated values of three existing correlations for the pool boiling heat transfer coefficient. The experimental variables, which mainly includes saturation temperature, heat flux and tube structure, have great influences on the correlation accuracy. Finally, to eliminate the influence of the experimental variable, a new correlation was proposed by introducing ratio of fin height to outer diameter of the tested tube. After the further verification, the proposed correlation can predict the heat transfer coefficient with the mean deviation of less than 10%.

Experimental study of dielectric liquid spray cooling on multi-scale structured surfaces inspired by leaf veins

Dielectric liquid spray cooling is a promising way to dissipate heat of high-power electronic devices. Surface modification is a most cost-effective method to enhance spray cooling. Inspired by leaf veins, this paper designs and fabricates macro-scale, micro- and nano- scale, and multi-scale structured surfaces for dielectric liquid spray cooling. The cooling characteristics are tested on a two-phase spray cooling system using HFE-7100. The results reveal that the heat transfer is enhanced on all the structured surfaces. Two bionic leaf vein structures, reticulated veins and parallel veins, are designed for macro-scale structured surfaces. The results show that the former one is superior to the other thanks to its better liquid distribution. For the micro- and nano- scale structured surfaces, due to the larger surface area and higher thermal conductivity, the graphene coating outperforms the carbon nanotube coating in heat transfer. Multi-scale structured surfaces, featured with leaf veins and micro- and nano- coatings, further enhance heat transfer. The heat flux increases by 116 % compared with that of the smooth surface. The evaporation efficiency reaches 60 % at the surface temperature of 80 °C. Furthermore, the effect of surface temperature on the enhancement ratio of heat transfer is analyzed, revealing various enhancement mechanisms of different scaled structured surfaces.

Improvement of an ionic wind blower’s flow characteristic by auxiliary electrodes for thermal management of light-emitting diodes

Electronic device advancements in power, size reduction, and integration have resulted in increased heat flux and operating temperatures, negatively impacting the reliability of electronics. Ionic wind cooling is a new, environmentally friendly, and energy-efficient thermal management approach that effectively cools high heat flux local heat sources. However, enhancing ionic wind strength continuously can be challenging. In this study, an ionic wind blower consisting of an emitting electrode, a collecting electrode, and auxiliary electrodes is constructed. The experimental verification confirms the supplemental acceleration capacity of the auxiliary electrodes. When determining the optimal operating voltage applied to the auxiliary electrodes, the power consumption of the system and the intensity of the output ionic wind are taken into consideration. Theblower’s operational and structural factors, such as the emitter structure, discharge gap, and distance between the auxiliary electrodes and collectors, are optimized according to how they affect the device’s functional qualities. The improved blower’s heat dissipation ability is evaluated by cooling an LED chip. The results demonstrate that the system performs optimally with an emitter having seven needles, a discharge gap of 5 mm, and 9 mm between the auxiliary electrodes and the collector. The wind speed reaches 2.47 m/s, while the power consumption is only 1.6 W. Compared to the absence of auxiliary electrodes (47.7 W/(m2∙K)), the system’s mean convective heat transfer coefficient can reach 61.12 W/(m2∙K), resulting in a temperature reduction of the LED chip by up to 41.6 °C. With increasing voltage, the heat transfer enhancement ratio improves, enabling a blower with auxiliary electrodes to provide significant cooling while consuming less power.

The behaviour of ternary hybrid nanofluid: Graphene oxide, Aluminium oxide, Silicon dioxide in heat transfer rate

The miniaturization in the design of the electronic system became inevitable due to the rapid advancement and development of technology. This has imposed challenges to the thermal management capability as the heat flux density has increased tremendously due to a smaller heat transfer surface. Nanofluids adoption in electronic cooling seems to be an alternative way for better heat dissipation. This research explores the feasibility of ternary hybrid nanofluids GO: Al2O3: SiO2 in water with different volume concentrations and mixture ratios in a serpentine cooling plate. In this research, 0.01% GO + Al2O3: SiO2, 0.006% GO + Al2O3: SiO2, and 0.008% GO + Al2O3: SiO2 in mixture ratios of 10:90 and 20:80 (Al2O3: SiO2) were studied. The result showed that 0.01% GO + Al2O3: SiO2 (10:90) nanofluids displayed the highest enhancement of heat transfer coefficient with 1.1 times higher as compared to the base fluid. This was then followed by 0.008% GO + Al2O3: SiO2 (10:90) and 0.006% GO + Al2O3: SiO2 (10:90) with 1.03 times and 0.87 times higher heat transfer coefficient enhancement consecutively as compared to the base fluid. In term of mixture ratios, GO in 10:90 (Al2O3: SiO2) performed better than 20:80. To assess the feasibility of adoption, the advantage ratio (AR) was conducted to measure both heat transfer enhancement and pressure drop effect. The AR analysis showed that at the lower Reynolds, Re number region, the 0.01% GO + Al2O3: SiO2 (10:90) ternary hybrid nanofluids was proven to be the most feasible due to a higher ratio of heat transfer enhancement over the pressure drop penalty.

Heat transfer performance and electrohydrodynamic characteristics optimization of a dividing-wall-type ionic wind heat exchanger

Heat exchangers play a major role in reducing emissions and conserving energy. There is a plethora of theoretical research on the enhancement of heat transfer caused by ionic wind, but there is still a lack of studies on the practical application of ionic wind in heat exchangers to increase heat exchange capacity. This work developed an original ionic wind heat exchanger with a dividing wall. It may be applied to difficult circumstances when conventional heat exchangers fail for air-to-air heat exchange. The study conducted experiments to investigate the impact of many factors, including intake air velocity, discharge spacing, and electrode distribution scheme, on the ionic wind intensity and the heat exchanger's increased heat transfer coefficient. A two-dimensional model was used to analyze the internal flow and thermal properties of the heat exchangers with wire emitters. By combining active and passive heat exchange technologies, an ionic wind heat exchanger with serrated grounded electrodes was presented. The outcomes demonstrated that a suitable intake air velocity enhances the heat exchanger's capacity for heat exchange. The improved heat exchange effect of ionic wind is more important when the inlet wind speed is less than 1 m/s. When the intake air velocity approaches 1.3 m/s, the ionic wind has less of an impact on the heat exchanger's capacity for heat exchange. The benefits of improved heat transfer are promoted by placing the emitter closer to the channel entry. Increasing the operating voltage further enhances the heat transfer enhancement ratio (HTER). The number and arrangement of wire electrodes should be chosen with consideration for the 'barrier effect' between neighboring emitters and the distance from the entrance when utilizing multiple wire electrodes. In comparison to the three-wire and five-wire electrodes, the four-wire electrode heat exchanger's HTER values were increased by 5.9 % and 14.3 %, respectively. The ionic wind heat exchanger's capacity for heat transfer is further increased using a zigzag grounding electrode. The zigzag uses a decreased aspect ratio, and the emitter is positioned slightly above the tip, improving the system's heat transfer capabilities. When compared to a heat exchanger with a flat plate grounded electrode, the four-wire electrode heat exchanger produced a 158 % increase in HTER value.

Numerical study on heat transfer and evaporation vaporization performance of solar assisted heat pump regenerative evaporator based on dual-phase change coupled heat transfer

Using phase-change slurry (PCS) as the working medium for solar energy systems, for example, in solar-assisted heat pump (SAHP), offers numerous advantages such as photovoltaic (PV) and photothermal (PT) energy generation and heat supply. The PCS experiences a dual-phase change coupled heat transfer during the heat release and refrigerant vaporization, as investigated through simulation. This study confirmed a coupled heat transfer with dual-phase change. It was found that there is more vapor at the upper coupling interface than at the lower coupling interface, and the difference is obviously enhanced when the flow rate decreases from 0.4 m s−1 to 0.1 m s−1 or the temperature increases from 12 °C to 16 °C. The increased rate of flow or decrease in refrigerant temperature is associated with a reduced equilibrium concentration at the lower coupling interface. Nevertheless, an increase in the flow rate at the lower coupling interface causes a decrease in the equilibrium concentration, while raising the temperature increases the concentration. When the flow rate increases from 0.1 m s−1 to 0.4 m s−1, the heat transfer coefficient at the upper/lower coupling interface increases from 159.77 W m−2 K−1/292.58 W m−2 K−1 to 593.54 W m−2 K−1/654.36 W m−2 K−1, and the heat transfer enhancement ratio reaches 45.39% and 9.29%, respectively. When the refrigerant temperature increases from 12 °C to 16 °C, the heat transfer coefficient at the upper/lower coupling interface decreases from 579.16 W m−2 K−1/572.91 W m−2 K−1 to 329.44 W m−2 K−1/365.67 W m−2 K−1, respectively. Increasing tilt angle of the pipe within a moderate range is a potential solution to enhance heat transfer and improve heat transfer uniformity.

A comparative study of internal heat transfer enhancement of impingement/effusion cooling roughened by solid rib and slit rib

In the present study, we conducted a conjugate heat transfer (CHT) analysis for double-wall cooling with impingement and effusion, incorporating various types of ribs, using the Reynolds-averaged Navier–Stokes (RANS) method and the modified shear stress transport (SST) turbulence closure model (SST-KIC), accounting for the Kato-Launder modification (K), intermittency (I), and crossflow (C) transition effects. We comprehensively discussed the impact of slit type (parallel, inclined, convergent, and divergent), open-area ratio (β = 5%, 20%, and 40%), and jet Reynolds number on the turbulent flow and heat transfer in a double-wall cooling with slit ribs. Our findings indicated that the introduction of slit ribs significantly improved heat transfer and its uniformity on the target wall, albeit with a slight increase in pressure loss. The overall Nusselt number and thermal-hydraulic performance (THP) in cases with slit ribs gradually decreased with β, yet remained up to 17% and 13% higher than those observed on a smooth target wall. Notably, the open-area ratio of the slit rib exhibited a more pronounced effect on heat transfer over the target plate. For the divergent slit rib within the Reynolds number range of 4000–16 000, the heat transfer enhancement ratio reached the highest value at β of 0.05. In addition, we computed the entropy production caused by fluid friction and heat transfer, as well as the overall entropy production in double-wall cooling at different β and Re. The analysis revealed that the slit rib target plate performed better than the solid rib target plate, showing a distinct advantage in terms of total entropy production.

Experimental and numerical study on thermofluidic characteristics of microchannel heat sinks with various micro pin-fin arrays arrangement patterns

Targeting at improving the comprehensive performance of microchannel heat sink, microchannel heat sinks integrated with various arrangements of micro pin-fins arrays are proposed in this paper. The thermofluidic characteristics and heat transfer enhancement mechanism of different pin-fins arrangement patterns is investigated by experimental test and numerical simulations. It is found that the partially uniform/gradient stagger pin-fins arrangement patterns present largest pressure drop with moderate heat dissipation performance, while the patterns with overall pin-fins distribution demonstrate superior heat transfer performance than all partial pin-fins distribution modes. Thus, overall gradient stagger pin-fins arrangement with moderate pressure drop yields optimal heat transfer capacity, achieving 4.8∼6.1 times enhancement than smooth channel. Furthermore, the partially uniform/gradient stagger pin-fins arrangement patterns indicate best temperature uniformity and thermal resistance with 26.2 K and 47.6% reduction than smooth channel at 80 ml/min, 100 W/cm2, respectively. The temperature trend in the longitudinal direction shows peaks and troughs for arrangements with partial pin fins distribution, while patterns of gradient pin fins distribution exhibit wavy curves of temperature rise. Additionally, the performance evaluation plot is adopted to describe the comprehensive performance of heat sinks with different pin fins patterns. It is observed that the overall gradient stagger pin-fins arrangement shows optimal comprehensive performance among all patterns, achieving heat transfer enhancement ratio greater than 4 in contrast with smooth channel. Finally, the correlations of Nusselt number and apparent friction factor for patterns are established. This study provides feasible insights on the performance improvement of microscale liquid cooling technologies for thermal management of electronic devices.

Numerical simulation of fin arrangements on the melting process of PCM in a rectangular unit

Innovative fin arrangements were proposed to enhance the thermal performance of latent heat storage units. The enthalpy-porosity approach was employed to optimize the fin distribution and predict the transient melting behaviors of N-octadecane. The unit with horizontal fins of uniform length was set as the base condition for comparison. It was found that the dead space of heat transfer and melting due to natural convection was not effectively eliminated by the conventional fin set. The region persisted at the bottom of the unit until the end of the melting process. In light of these observations, the fins were repositioned and resized. Three alternative fin configurations were proposed and evaluated: angled fins, asymmetrically located fins, and stepped fins. Results illustrated that fin structures significantly influenced the heat transfer characteristics and melting behaviors of latent heat storage units. Generally, greater enhancements were attained with downward angles and downward offset distances. The downward angle −20°led to an accelerated melting rate of 18.3 % compared to the base condition. In case 9, where the fins were moved down by 12 mm, the heat transfer enhancement ratio reached 32.5 %. However, introducing inhomogeneity in the fin length had only a minimal impact on the melting process.

Outlining the impact of discrete filling of metal foams on thermodynamic performance

The desire of this paper is to present the numerical analysis of thermodynamic performance of high porosity metal foams with discrete filling inside the horizontal pipe. The heater is embedded on the pipe’s circumference and is assigned with known heat input. To enhance heat transfer, aluminum metal foam of 10 PPI (pores per inch) with porosity 0.95 is filled into the pipe. In filling, two kinds of arrangements are made. In the first arrangement, the metal foam is filled adjacent to the inner wall of the pipe (Model (1–3)), and in the second arrangement, the metal foam is located at the center of the pipe (Model (4–6)). So, six different models are inspected for different fluid velocities (0.7–7 m/s) under turbulent flow conditions. Darcy Extended Forchheimer (DEF) is combined with local thermal non-equilibrium (LTNE) models for estimating the flow features and heat transfer via metal foams and initially validated with experimental outcomes. The application of the second law of thermodynamics via metal foams is the novelty of current investigation. In the study, the thermal performance is assessed in terms of heat transfer enhancement ratio and heat transfer performance ratio, and the thermodynamic performance is evaluated based on exergy and irreversibility analysis. In the analysis, the parameters like mean exergy based Nusselt number (Nue), heat transfer and flow frictional irreversibility are considered for the evaluation. The overall thermal performance and exergy transfer found higher in Model (1–3) than the Model (4–6) respectively. The flow and heat transfer irreversibility found less in Model (1–3) than the Model (4–6) respectively. The superior thermodynamic performance is obtained from the metal foam filled cases than from the clear pipe. In that, Model-2 suits the best configuration for designing the thermal system.

Forced convection heat transfer characteristics of aviation kerosene in a horizontal tube under electric field

The electrohydrodynamic (EHD) technique can considerably improve the performance of aviation heat exchangers. In this study, the forced convection heat transfer characteristics of aviation kerosene in a horizontal wire-tube structure under electric field were investigated. The influences of positive and negative high voltages on the heat transfer in test section were compared. Key factors affecting EHD heat transfer enhancement, such as fuel inlet temperature, mass flow, heat flux and operating pressure were explored. Furthermore, the cause of electrode corrosion was investigated using Scanning Electron Microscope (SEM) and Energy Disperse Spectroscopy (EDS). The experimental results showed that the circumferential temperature distribution of the test section was uneven owing to buoyancy in the absence of electric field. As the applied voltage increased, temperature heterogeneity decreased. Negative voltage improved the heat transfer more effectively than positive voltage. The electric field enhanced the heat transfer of high-temperature fuel more effectively. In the charge injection mechanism voltage range, the numerical simulation results were more similar to the negative voltage experimental results. The heat transfer enhancement ratio was reduced by an increase in the fuel mass flow and operating pressure. A higher heat flux led to a more substantial buoyancy effect, which inhibited the electric field force effect. Finally, the corrosion rate of positive high voltage electrode was the highest, resulting in rapid loss of metal elements from the electrode surface.

Numerical Research on the Heat-Transfer Enhancement of Printed Circuit Heat Exchanger in the Presence of Pulsating Flow

Abstract This paper presents a numerical investigation on the flow and heat-transfer characteristics of liquid lithium and helium in printed circuit heat exchanger (PCHE) channels with pulsating flow introduced. The effects of the mass flowrate of helium, frequency, and amplitude of pulsating flow were examined, respectively. The findings indicate that the introduced pulsating flow has a positive effect on heat transfer at a lower Reynolds number. With the increase of pulsating frequency, the heat-transfer enhancement may decline, and the pressure drop is reduced, with Colburn factor (j) and Fanning friction factor (f) varying marginally. In the flow state with enhanced heat transfer, keeping a lower pulsating frequency, the heat-transfer enhancement ratio E(h) exhibits an approximately linear increase with the pulsating amplitude. Meanwhile, the average pressure drop, Colburn factor (j) and Fanning friction factor (f) also increase with the amplitude, with the fluctuation amplitude of each parameter directly proportional to the amplitude.

Performance Evaluation Based on Exergy Analysis Through Partially Filled Metal Foams in Forced Convection

Abstract The intention of this paper is to present the numerical analysis of thermal performance and exergy transfer through high porosity metal foams filled partially in a horizontal pipe. The heater is embedded in the pipe's circumference and is assigned with known heat input. To enhance heat transfer, aluminum metal foam of pore density 10 with porosity 0.95 is inserted adjacent to the pipe's inner wall. To determine the optimal thickness of metal foam for enhancing its performance thermodynamically, metal foams with five different thicknesses (10, 20, 40, 60, and 80 mm) are examined in this research for a fluid velocity ranging from 0.7 to 7 m/s under forced convection heat transfer condition. The Darcy– Brinkman–Forchheimer and local thermal nonequilibrium (LTNE) models are used for forecasting the flow features and heat transfer through the metal foams, respectively. The numerical methodology implemented in this research is confirmed by comparing the present outcomes with the experimental outcomes accessible in the literature and found a fairly good agreement between them. The thermal performance is assessed in terms of heat transfer enhancement ratio and performance factor, and the thermodynamic performance is evaluated based on exergy analysis. In the exergy analysis, the parameters like mean exergy-based Nusselt number (Nue), merit function (MF), and nondimensional exergy destruction (I*) are considered for the evaluation. The result shows a better performance from partially filled metal foams than from completely filled metal foams.

Analysis of the laminar flow and enhanced heat transfer rate through a triangular array of cylinders embedded in a fluid-saturated porous media with mixed convection

An investigation of the laminar flow and enhanced heat transfer rate through a triangular array of cylinders embedded in a fluid-saturated porous media is considered. Mixed convection covering forced convection cases has been studied using a finite-volume approach considering the local thermal equilibrium model. Darcy–Brinkman–Forchheimer momentum equations are used to characterize the porous media in question. The study is constrained to the low and intermediate Peclet number (Re = 5 to 40, Pr = 50) and high range of Darcy number (Da = 10−3 to 10−1) for different cylinder spacings (0.7 ≤ φ ≤ 0.99). It is anticipated that porous media would enhance heat transfer, but it derives a multiple order in pressure drop indubitably not desirable in many heat applications. Investigation reveals that both mean drag coefficient (CD ) and Nusselt number (Nu) are strong functions of Da and φ; however, the influence of the buoyancy parameter is mainly witnessed in higher permeability levels and high Peclet numbers. The study also details the effect of governing parameters on mean gap velocities and pressure coefficients. Surprisingly, recirculation wakes exist for the lowest cylinder spacing (φ = 0.7) in high fluid momentum. Low φ and high permeability are desirable in a forced convection regime, whereas both φ and Da should be high in the case of mixed convection. This article also quantifies the combined effect of the Darcy number and buoyancy parameter on the heat-transfer enhancement ratio. A maximum of 112% increase in Nu for φ = 0.7 and 452% for φ = 0.99 are reported at Da = 10−1, but at the cost of higher pressure drop in the latter case.

R245fa flow boiling heat transfer in a sintering and electroplating modulated tube

• R245fa flow boiling heat transfer is investigated in a modulated tube coated by using sintering and electroplating technology. • Flow patterns and dynamic liquid height analyses are proposed to explore the heat transfer enhancement mechanisms. • The micro-nano scale coating on the tube inner wall could modulate flow pattern to enhance heat transfer. • The maximum EF and PEC can reach 2.87 and 2.79. The flow boiling performance of R245fa was investigated in a bare stainless-steel tube (BT) and a sintering and electroplating modulated stainless-steel tube (SET). The test tubes had the same inner diameters of 10.02 mm and effective heat transfer lengths of 800 mm. The mass fluxes, inlet vapor mass qualities and heat fluxes were in ranges of 192.61-705.35 kg/(m 2 ·s), 0.01–0.9 and 4.99-74.97 kW/m 2 , respectively. Three flow patterns were identified in both BT and SET. The annular flow pattern conversion line of SET was earlier than that of the BT. With the increase in vapor quality, the boiling heat transfer coefficient increased and then decreased at q≤ 45 kW/m 2 , whereas it decreased at q ≥60 kW/m 2 . For the BT, the heat transfer coefficient was weakly affected by the mass flux, but it increased with the increase in mass flux for the SET. For the BT and SET, the frictional pressure drop increased and then decreased with the increase in mean vapor quality at a certain heat flux. Owing to the strong wettability of SET, the thick liquid film at the tube bottom in stratified flow was forced to the tube top and the liquid droplets in annular flow were captured by the surface coating. Thus, flow patterns were modulated to enhance heat transfer. The maximum of the heat transfer enhancement ratio and performance evaluation parameter can reach 2.87 and 2.79, respectively.

Investigations of mixing and heat transfer in a structured tube‐in‐tube millireactor by numerical, experimental and statistical methods

AbstractMillireactors are widely used in flow chemistry due to their process intensification performance and sufficient product throughout. A novel structured tube‐in‐tube millireactor with industrial potential was designed, whose mixing and heat transfer were investigated by experiment and CFD. Partial least square regression (PLS) was introduced in the flow chemistry study and used to determine that the key parameters of the millireactor were jet Reynolds ( ) and hole diameter . The introduction of twist tape in the annular space generates swirling flow, which effectively increases the heat transfer capacity (up to 16.5 times). This millireactor achieves excellent mixing (&gt;99% of mixing intensity after 1 cm) and the heat transfer enhancement ratio up to 6.5. The local flow field and concentration field from CFD model agreed with those obtained from PIV and PLIF visualization experiments.

Effect of Utilizing Staggered Semi-Porous Fins on Channel Flow Heat Transfer Enhancement

A comprehensive numerical investigation of fluid stream and laminar forced convection were conducted to simulate the effects of the semi-porous fins, which are a new combination of porous and solid parts, inside a partially heated channel with a staggered arrangement. The present numerical approach is based on the Navier-Stocks and Darcy-Brinkman-Forchheimer equations in the fluid and semi-porous regions, respectively. The effects of various parameters such as Reynolds number, Darcy number, fin height, and aspect ratio of the porous region’s height to total fin height were studied. Also, the effects of these parameters are illustrated further as a variation of dimensionless pressure penalty and heat transfer enhancement ratio in the flow domain. The results demonstrate that using a new type of fin with both solid and porous parts in a single fin, enhances the heat transfer up to 6% compared to the porous fin and decreases the pressure penalty up to 23% compared to the solid fin at Darcy number of 10−4 when the percentage of aspect ratio is equal to 20%. Furthermore, it was found that the Darcy number of 10−5 is a critical value for the optimum utilization of semi-porous fins.

Combined effect of electric field and nanofluid on bubble behaviors and heat transfer in flow boiling of minichannels

In this work, electric field and nanofluid are coupled to enhance flow boiling heat transfer in minichannel. Pin electrodes are adopted to create non-uniform electric field. The effects of nanoparticle mass concentration, surfactant type and mass concentration under electric field on nanofluid flow boiling heat transfer are studied. Image analysis technique is used to quantify boiling bubble behaviors. The results demonstrate that without electric field, bubbles for nanofluid flow boiling are smaller and more discrete than R141b. Under electric field, bubble size reduces and the number of bubbles increases, which reveals synergy effect of electric field and nanofluid on bubble behaviors is effective. Besides, electric forces mitigate bubble coalescence and delay transition from bubbly to slug flow. Maximum heat transfer coefficient of nanofluid is 80% higher than R141b. Under electric field, boiling heat transfer of nanofluid modified by anionic surfactant (SDBS) is better than cationic (CTAB) and nonionic (Span80) surfactant. • Electric field and nanofluid were devised to enhance flow boiling of minichannel. • Bubble behaviors and flow pattern transition in nanofluid flow boiling under electric field was discussed. • Synergy effects of electric field and nanofluids on enhancing flow boiling heat transfer was discussed. • Maximum heat transfer enhancement ratio of 1.8 for nanofluid and electric field is obtained.

Flow friction and thermal performance of dimple imprinted based solar air-heater: A numerical study

The thermal and flow characteristics of dimple imprinted absorber plate solar air-heater (SAH) are investigated numerically in this study. A forced-convection heat transfer mechanism is adopted to speed up the heat transfer rate between the air and the heated surfaces of the SAH. The three-dimensional simulations were run with ANSYS FLUENT (V-15) CFD code and the SST k-ω turbulence model, which closes the turbulence equation. By altering design variables such as pitch ratio (transverse pitch to longitudinal pitch) and Reynolds number, the thermal performance of staggered and inline dimple imprinted absorber plate, in SAH has been evaluated. Five alternative pitch ratios and Reynolds numbers ranging from 2500 to 12500 are used for each staggered and inline dimple imprinted absorber plate arrangement. A constant input heat flux of 1000 W/m2 to the absorber plate is used in the numerical analysis. The impact of pitch ratios and Reynolds numbers on a dimensionless number such as Nusselt number (Nu), friction factor (f), Nusselt number ratio (Nu/Nu 0), friction factor ratio (f/f 0) and thermo-hydraulic performance parameter (THPP) is investigated and addressed. The dimpled surface's heat transfer rates are measured and compared to the smooth surface’s result. When compared to the smooth duct; the dimpled imprinted absorber plate SAH demonstrates a considerable improvement in heat transfer and friction characteristics. At dimple pitches of S T/D = 2.5 and S L/D = 2, the heat transfer enhancement ratio of dimpled surfaces is around 1.597 times higher than the smooth surface in the staggered arrangement. At dimple pitches of S T/D = 2 and S L/D = 2.25, the heat transfer enhancement ratio for dimple surface is roughly 1.660 times better than the smooth surface in an inline arrangement.