Heat Transfer Enhancement Effect Research Articles

Enhancing the thermal performance of diffusion absorption refrigeration system by using magnesium aluminate spinel oxide compound nanoparticles: an experimental investigation

This article aims to enhance the efficiency of Diffusion Absorption Refrigeration (DAR) systems which operate at low efficiency. This study experimentally investigates the effect of passive heat transfer enhancement by using various nanofluids as working fluids including ammonia/water couple with different rates of magnesium oxide aluminate spinel (MgOAl2O3) particles. The tests have been carried at the same ambient conditions to obtain reliable results. Additionally, nanofluid has been prepared and tested in two different particle concentration (1% and 2%). Adding nanoparticles to the base fluid (25% ammonia/water) can make an important positive effect in increasing heat transfer by extending surface area and heat capacity of the fluid. Experimental results showed that using nanofluid as working fluid in DAR system results in faster evaporation which decreases operation time and increases heat transfer in generator. Moreover, the DAR system containing 2% concentration of nanofluid solution led to a 37.4% increase in performance coefficient (COP) and a 44.2% increase in exergetic performance coefficient (ECOP) compared to the DAR system without nano particles.

Research on Heat Transfer Enhancement of Vortex-Induced Vibration of Side-by-Side Double Tubes

The fluid-induced vibration heat transfer enhancement technology has great potential, because it can effectively improve the heat transfer efficiency without consuming additional energy. A reasonable design of the elastic heat exchange tube bundle structure is the key to the engineering application of this technology. In this paper, the effects of flow velocity and the center distance on the characteristics of vortex- induced vibration heat transfer in side-by-side double circular tubes are studied. We demonstrate that, when the center distance is no more than (outer diameter), the interference effect of the flowfield around the two tubes is greater, and the amplitude of the double tube is small. However, with the increase of , the flowfield interference decreases. Within the scope of this paper, two modes of vibration are obtained: in-phase vibration and out-of-phase vibration. When and , the fluid vortex-induced elastic supported side-by-side double circular tubes’ vibration can enhance heat transfer. Especially, when , the heat transfer enhancement effect is the strongest, reaching 14.5%. However, when , the heat transfer enhancement effect is not obvious. The research on the heat transfer characteristics of fluid flowing around the static double tube shows that when the distance is small, the flow velocity around the static tubes is larger, resulting in a higher surface average heat transfer coefficient, and as the distance increases, the heat transfer coefficient decreases.

Flow-induced oscillation of membrane tapes and its effect on heat transfer of a transitional pipe flow

Flow-induced oscillation is an effective way for heat transfer enhancement, which embraces the advantages of requiring no extra energy consumption and better performance for preventing fouling or soot formation. In the present study, a thin membrane tape with length, width and thickness of 90 mm, 3.8 mm and 13.3 um respectively was mounted inside a horizontal pipe with an internal diameter of 13 mm. By visual measurement, the tape was demonstrated to be able to oscillate within airflow and the oscillation frequency was found to increase with the flow rate. Constant heat flux was applied at the pipe wall through electrical heating and the wall temperature was obtained by thermocouples. In low Reynolds number turbulent region (Re between 1800 and 4200), the mean heat transfer coefficient and flow resistance were found to be 1.3-1.7 times and 1.6-2.1 times than that of a smooth tube, respectively. However, the heat transfer enhancement effect is most pronounced at lower Re and as the Re increases, this effect becomes weak. Nevertheless, the spontaneous oscillating tapes can be a very promising heat transfer intensification element. This study is a primary investigation attempt and flow-induced oscillations of membrane tapes of different dimensions and the use of multiple membrane tapes are awaited to be studied deeply.

Investigation of the heat transfer characteristics of a novel thermosyphon with different particle sizes

In this study, the fluidized bed heat transfer technology is introduced into two-phase closed thermosyphon (TPCT) to build a novel thermosyphon system, which aims to improve the heat transfer performance of TPCT. A series of experiments are conducted to investigate the joint effects of particle size, amount of added particles and heating power on heat transfer characteristics of the novel thermosyphon. Results show that the heat transfer performance of TPCT is significantly improved in the evaporator section because of the addition of silicon carbide particles. However, in the condenser section, the existence of particles leads to the deterioration of heat transfer. In general, the heat transfer enhancement effect of the large size particles is superior to that of the small size particles. The overall thermal resistance is reduced because of the addition of large size particles. The 3D diagrams are established to assess the joint effects of various parameters.

Heat transfer and flow characteristics of Fe3O4-water nanofluids under magnetic excitation

Numerical simulations were adopted to explore the heat transfer and flow characteristics of magnetic nanofluids under different magnetic field intensities, volume fractions, and magnetic field directions. Due to these parameters have a significant effect on thermal hydraulic performance and the mechanism of this enhancement are not yet clear, a turbulent model of Re-normalization group (RNG) k-ε is used to analyze the variation of thermal boundary layer and particle motion. The obtained results found that the effect of heat transfer enhancement was small under a weak magnetic field, but it increased considerably under a strong magnetic field. Besides, the convection heat transfer coefficient initially increased and then decreased with an increase of volume fractions of magnetic nanoparticles. Moreover, the directions of the magnetic field have a significant effect on convective heat transfer coefficient, which results in 8% increase when the direction is perpendicular to direction of flow.

Effect of enclosure on heat transfer characteristics of dual swirling flame impinging on a flat surface

Experimental investigations have been carried out for impingement heat transfer characteristics of an enclosed dual swirling flame for various enclosure configurations. Effects of the size of the enclosure (DE = 70 mm, 90 mm, 90 mm, 110 mm and 150 mm) and the level of gap between the top of the enclosure and the target surface (G = 12, 16, 20, 24 and 28 mm) have been examined at fixed other operating conditions (Re(o), H/Dh and S). In addition to this, the effects of variation in outer swirling flame Reynolds numbers (Re(o) = 7000–13,000), dimensionless separation distances (H/Dh = 2–5) and the swirl number (S = 0.86, 1.54 and 2.4) have also been examined. Impingement heat transfer characteristics strongly depend on the size of the enclosure (DE) and the level of gap (G). Larger sized enclosures (DE of 110 and 150 mm) with smaller gap levels (G of 12 and 16 mm) perform much better than small sized enclosures (DE of 70 and 80 mm) with larger gap levels (G of 24 and 28 mm). Heat transfer performance continuously increases with an increase in Re(o) in presence of an enclosure in contrast to unenclosed flames where performance deteriorates at high Re(o). The heat transfer enhancement effect in presence of enclosure is more pronounced at larger separation distances (H/Dh of 4 and 5) because of the formation of the outer recirculation zone (ORZ). The effect of the presence of enclosure on impingement heat transfer enhancement becomes more significant at higher levels of swirl.

Promising dimple technologies of vortex heat and mass transfer enhancement in energy and microelectronics

Turbulent separated air flow and heat transfer at the starting hydrodynamic length of the longitudinal section of the dimpled narrow channel (1 in height, 4 in width, and 84 in length) with inclined one-row oval-trench dimples (OTD) are calculated. Packages of 20 and 31 dimples are compared. Each dimple has a length of 4.5, a width of 1, a depth of 0.2, an edge rounding radius of 0.2, and an inclination angle of 45°. Re=6000 and is determined in terms of the flow velocity at the channel entrance. The effect of abnormal separated flow intensification and heat transfer enhancement in OTDs due to the dimple density is confirmed. (fx/fxpl)min=−4 and (Nu/Nupl)max=7.2. Relative heat transfer grows by a factor of 1.9 and relative hydraulic losses – by a factor of 1.6.

Research on influencing factors of heat transfer enhancement fins in fuel cell cooling channel

PEMFC gradually entered the field of vision of scholars and aroused widespread attention and research, because it has the characteristics of high efficiency, no pollution, and low noise. Traditional PEMFC thermal management system has low efficiency and poor temperature distribution uniformity. This article innovatively proposed a method of using enhanced heat exchange fins for PEMFC thermal management system and established a multi-field coupling simulation model to study the influence of the design parameters of the enhanced heat exchange fins on the temperature of PEMFC, and the optimal fin design scheme is proposed. The results indicated that the fins have an excellent heat transfer enhancement effect, effectively reducing the temperature of each sub-component of PEMFC. Otherwise, different fin angle and fin length have a positive effect on the performance of the thermal management system. Then the design experience of this article can be provided to scholars for reference.

Effective heat transfer surfaces of tubes and plates with spiral vortex generators – inclined oval-trench dimples

Numerical simulation of laminar heat transfer enhancement at the hydrodynamic stabilization length of the tube with 8 inclined longitudinal oval-trench dimples (OTDs) in the transformer oil flow has revealed an optimal inclination angle of dimples, at which a value of thermal and hydraulic performance (THP) is achieved. Heat transfer of a dimpled tube increases by a factor of 28 in comparison with a smooth tube at practically invariable hydraulic losses. Heat transfer enhancement is calculated in turbulent air flow around a flat plate with a package of 16 one-row dimples inclined at an angle of 60° on the longitudinal section of 40 in length and 4 in width under the symmetry conditions at the side boundaries of the plate section and at a dimple step of 2.4. The dimple is 1 in width, 4.5 in length, and 0.2 in depth. Its rounding radius is 0.3. The boundary layer thickness at the section inlet is 0.175. The effect of abnormal separated flow intensification and heat transfer enhancement in dimples has been confirmed. (f/f pl )min=-3, (Nu/Nu pl )max = 4. Relative heat transfer grows by a factor of 1.43 in comparison with the smooth plate and the drag coefficient increases by a factor of 2.08. However the thermal and hydraulic performance defined in terms of heat load growth is 1.12.

Investigation of Heat Transfer Enhancement Effect on Normal and Nano Coated Wick Structure Heat Pipes-A Comparative Assessment

Removal of heat generation is an important characteristic needs to be considered in electromechanical and electronic devices which improve the stability and feasibility of system. Despite numerous cooling methods, heat pipes are recent updating in research line. Heat pipes are one of the super conducting medium of heat energy and it is being used as an equipment to absorb more heat through phase change process of cooling medium circulated in it. It ensures the direct enhancement in heat transfer capacity and characteristics. Nowadays, improvement of the thermal performance in heat pipes getting up with various technologies, especially combination of heat pipe and Nano fluids. It has been experimentally practiced and various results are observed by previous researches that wick structure also a part of reason in improvement. The aim of this research work is to analyze the influence of wick material to improve heat transfer characteristics in heat pipes. In addition, combination of nano coated wick material with heat pipes is comparatively analyzed. From the final observed results it was found that, the best combination of wick material is supporting the better cooling requirements in electronic devices.

Study on the flow and heat dissipation of water-based alumina nanofluids in microchannels

In this paper, the flow and heat dissipation performance of 0.1–0.5 vol per cent of Al2O3 -water nanofluid through serpentine micro-channel was experimentally studied in the Reynolds range 124–1000. By analyzing the experimental results of nanofluid flow and heat dissipation: In the experiment, the Nusselt number of nanofluids is 1.12–1.66 times that of deionized water, which indicates that Al2O3 -water nanofluids have significant heat transfer enhancement effect. Nanofluid flow resistance is 1.02–1.80 times that of deionized water, this indicates that Al2O3 -water nanofluid has the disadvantage of increasing energy consumption. From a comprehensive performance point of view: all the nanofluids used in the experiment have enhanced heat transfer factors more significant than one, and the enhanced heat transfer effect is better than that of deionized water, this suggests that Al2O3 -water nanofluid has the drawback of rising energy consumption. All the nanofluids used in the experiment have enhanced heat transfer factors more significant than one, and the enhanced heat transfer effect is better than that of deionized water. Nano-fluids, with a volume fraction of 0.4%, have an average increased heat transfer factor of 1.4, which is the best overall performance.

CFD analysis on flow and heat transfer characteristic in natural circulation system with subchannels under rolling conditions

The combination of advancement in nuclear technology and ship engineering is a new technology of nuclear application which can not only marine capability and provide power for remote islands, but also help solve the practical issues of high cost, high pollution and high pressure in cutting down emissions in the petroleum exploitation field. The CFD analysis in this paper focuses on the exploration of single-phase natural circulation flow and heat transfer characteristics for rod bundle under rolling conditions. The rolling version CFD tool is developed and compared with the calculated results by system code. The natural circulation system which consists of a 2 by 2 subchannel is established. This paper mainly focuses on the effect of different rolling parameters on natural circulation flow and heat transfer characteristics for rod bundle loop under rolling motion. The thermal–hydraulic performance varies periodically under rolling motion. The variation period equals to rolling period. The stronger the rolling intensity is, the greater the fluctuations of different thermal–hydraulic parameters are. The average values of mass flux and heating section temperature difference under rolling conditions are lower than those under non-rolling conditions, proving that rolling motion may enhance heat transfer. Meanwhile, the inlet temperature of heating section under rolling conditions is lower compared with that under non-rolling conditions. With increasing rolling intensity, the effect of heat transfer enhancement is greater.

Experimental study on heat transfer and flow structures of feedback-free sweeping jet impinging on a flat surface

This study examined the flow structure and heat transfer characteristics of a sweeping jet generated from a feedback-free fluidic oscillator. A feedback-free sweeping jet (FFSJ) was proposed to enhance impinging heat transfer with a large jet-to-wall spacing. The surface temperature field of a plate cooled by a jet impingement was measured by thermographic phosphor thermometry using the decay-slope method according to a range of Reynolds number (Re) and various jet-to-wall spacing (Z/dh). The results were compared with those of a square jet (SJ) and conventional feedback sweeping jets (FSJ). Time-resolved particle image velocimetry, proper orthogonal decomposition analysis, and smoke visualization were conducted to determine the external flow characteristics of the FFSJ. The FFSJ oscillated with the 7dh wavelength. The frequency and sweeping angle were higher and smaller, respectively, that of the FSJ. The heat transfer performance of an impinging FFSJ was superior to that of an SJ when the jet-to-wall spacing was lower than 5dh. The peak Nusselt number increased with increasing Reynolds number. At a fixed Reynolds number of 32,000, the local heat transfer performance of the FFSJ was superior to that of the FSJ. As the jet-to-wall spacing increased, the effect of heat transfer enhancement also increased. At Z/dh = 8, the peak Nusselt number of the SJ surpassed the FFSJ, but the integrated heat transfer rate of an FFSJ was comparable to that of an SJ.

Pore-scale investigation on the heat-storage characteristics of phase change material in graded copper foam

The mechanisms responsible for the effect of metal foam, having a varying degree of porosity, on the phase change process in a phase change material (PCM) are not clearly understood. In this work, the pore-scale heat storage performance of the graded metal foams saturated with paraffin has been investigated using the computational fluid dynamics (CFD) method. The results demonstrate that the metal foam with a graded porosity accelerates the heat storage of PCM. In terms of the gradient dimension of the metal foam, the full melting time of the negative model 2 has been observed to be shortened by 2.6% as compared to the uniform model 1 and reduced by 15.5% as compared to the positive model 3. This is because the smaller the porosity of metal foam is, the faster the melting conducts. For the negative model, the relatively small porosity region is closer to the heat source than the large porosity region, which makes the heat transfer enhancement effect more evident than the weakening effect. Meanwhile, the full melting time shows an initial increasing trend, followed by a decrease for an increase in the porosity gradient difference. The optimal gradient difference is found to be −0.12 at the average porosity of 0.86 in this study.

Heat transfer in a liquid under focused ultrasonic field

The ultrasonic heat transfer enhancement has a strong dependence on the sound intensity. The higher the local sound intensity is, the better the heat transfer enhancement achieves. Therefore, a focused ultrasonic field is proposed in cavities with elliptical cross-section based on interference criterion and standing wave criterion. To design a cavity with focused ultrasonic field, sound source and heat source are purposely located at the two focus points where sound intensity could be reinforced. There is a compromise between the value of the standing wave coefficient and the time-averaged sound pressure. The profile of heat transfer coefficient is found to be spatially consistent with the simulated time-averaged sound pressure distribution. The cavities with focused ultrasonic field always present larger heat transfer coefficient of natural convection heat transfer than the ordinary ones and the cavity with standing wave coefficient k2 = 8 presents better natural convection heat transfer performance than the cavity with standing wave coefficient k2 = 6. For the sub-cooled boiling heat transfer, bubble plays an important role on the ultrasonic heat transfer enhancement effect. At larger heat flux or smaller sub-cooled temperature, bubbles gather together to form bubble cloud that presents large thermal resistance. The cavitation collapse can disperse the bubble cloud.

Development of micro/nanostructured-Cu-TiO2-nanocomposite surfaces to improve pool boiling heat transfer performance

In the current study, a new single-step forced electrodeposition method is proposed for the fabrication of intended micro/nanostructured surfaces. In this method, high thermal conductive Cu-TiO2 nanoparticles (10 nm to 20 nm) are deposited on copper sample with higher current density followed by sintering at reducing atmosphere. Initially, the nanoparticles are deposited uniformly on the copper sample by employing single-step forced convection electrodeposition. Finally, a single-step sintering is employed to enhance the adhesion of the coated layer with the copper sample. During sintering (annealing), the growth in nano-grains is occurred which enhances the inter-connectivity between the grains and also between the grains and base copper surface. The coating thickness (19 to 42 μm), porosity (43 to 75%), wettability (65° to 38°), and roughness (0.38 to 1.32 μm) of the developed surfaces are increased with increase in current density during the electrodeposition. The electrodeposition is a simple and cost effective method and able to provide more dense and stable surfaces. After the surface characterization, their heat transfer performances are analyzed through pool boiling experiments. The maximum percentage reduction in boiling incipience temperature, maximum percent augmentation in critical heat flux and boiling heat transfer coefficient on coated surfaces are 66.7%, 95.71%, and 317%, respectively. Five repeated experimentations are performed at a period of 21 h. The highest variations of ±8.6% in CHF and ± 2.1 °C in wall temperature are observed. Heat transfer enhancement mechanism and effects of various parameters on pool boiling are discussed. The present outcomes are also compared with the published outcomes.

Performance optimization for shell-and-tube PCM thermal energy storage

Abstract Phase change material (PCM) based latent heat thermal energy storage (LHTES) systems have been a technology of increasing interests. Extensive experimental and numerical studies on the heat transfer enhancement of PCM have been reported. However, a thorough analysis of the effects of PCM heat transfer enhancement in combination with geometric optimization on the overall storage performance of an LHTES system is still lacking. In this work, a performance index, the effective energy storage ratio Est, based on the effectiveness-NTU theory, which set up a standard to compare TES systems, was adopted to evaluate the effective energy storage density of an LHTES system. Using the conjugate heat transfer analysis, we investigated the impact of the key parameters and flow conditions, including the geometric parameters (tube length-diameter ratio L/di, PCM volume ratio λ), turbulent versus laminar flow conditions of HTF, and the effective PCM thermal conductivity keff, on the performance indicator Est. It was found that the effective energy storage ratio increases with tube length-diameter ratio, and an optimal PCM volume ratio exists. Increasing the effective PCM thermal conductivity is only effective in enhancing the effective energy storage ratio when PCM volume ratio is above certain value. Over 500 sets of parametric studies were performed to optimize the PCM volume by maximizing the effective energy storage ratio. The results show that for both laminar and turbulent flow, optimal PCM volume ratio and maximal effective energy storage ratio increases with tube length-diameter ratio and effective PCM thermal conductivity. The enhancement of effective PCM thermal conductivity only noticeably increases maximal effective energy storage ratio when tube length-diameter ratio is above a certain threshold, i.e., around 800 for laminar flow and around 600 for fully turbulent flow. The fully turbulent flow greatly enhances the charging rate by 50 times and increases the capacity effectiveness at optimal PCM volume ratio to 0.6-0.9 from 0.2-0.8 at laminar flow conditions, but the maximal effective energy storage ratio of fully turbulent flows are generally lower than those of laminar flows. This work is the first systematic numerical analysis of the complete ensemble design factors of a shell-and-tube LHTES system, and it is recommended that the method could be a standard one for the engineering design of an LHTES system.

Experimental research on heat transfer and flow resistance properties in spiral twisted tube heat exchanger

An experiment was conducted to gain the heat transfer and flow resistance characteristics of a high-viscosity thermal oil and water in spiral twisted tube heat exchanger. Experimental parameters covered the oil and water mass fluxes of 1000–3000 kg·m−2·s−1, 160–1200 kg·m−2·s−1, respectively; The water and oil inlet temperature of 15℃/60℃, respectively; And shell-side Re < 2050 and tube-side 12,000 < Re < 45,000. In addition, the heat transfer and flow resistance properties of the spiral twisted tube heat exchanger was compared with that of tube-shell heat exchangers of the ordinary and spiral-groove tube. The experimental results suggested that the spiral twisted tube heat exchanger demonstrates an obvious heat transfer enhancement effect in both laminar and turbulent flows than that of two tube-shell heat exchanger. And the experimental data of the heat transfer and flow resistance properties of the spiral twisted tube were compared to the classical and references correlations. Finally, the fitting heat transfer correlation of spiral twisted tube heat exchanger was gained within a deviation of ±10%.

Effects of Channel Geometries and Acceleration on Heat Transfer of Hydrocarbon Fuel

The convective heat transfer enhancement and deterioration of the supercritical hydrocarbon fuel in the regenerative cooling channels of a scramjet were investigated numerically. This study focused on the heat transfer enhancement and deterioration suppression of the spiral rectangular channels with enhanced heat transfer effects resulting from the enhanced turbulence intensity and induced spiral flow. As a result, compared with the straight rectangular channel, the maximum temperature of the spiral rectangular channel with a pitch of 20 mm was reduced by 31.3%. Although the cross-sectional shape and heat transfer area of the rectangular channel and the spiral rectangular channel were the same, the spiral rectangular channel with a short pitch had significant effects of heat transfer enhancement and deterioration suppression. Furthermore, the influence mechanisms of the acceleration field on the convective heat transfer were explored. Results indicate that acceleration has a significant effect on the heat transfer of the supercritical hydrocarbon fuel in the straight rectangular channel, but has little influence on the heat transfer of the spiral rectangular channel with a pitch of 20 mm.

Investigation into convective heat transfer of nanofluids at the thermal boundary layer development region in micro-channels

The convective heat transfer at the thermal boundary layer development region in microchannels was theoretical and experimently investigated with de-ionized water and Al2O3-H2O nanofluids as the working fluids. It was found that nanofluids could effectively decreased the wall temperature and improve the heat transfer coefficient compared with the de-ionized water at the thermal boundary layer development zone. When the serious agglomeration of nanoparticles didn’t appear, increasing the concentration of Al2O3 nanoparticles could effectively increase the heat transfer enhancement effect. And the local heat transfer coefficient increase amplitude by using the nanofluids decreased along the direction of refrigerant flow. At the thermal boundary layer development zone, higher inlet velocity would get the larger increase amplitude of the heat transfer coefficient. At the thermal boundary layer development region, lower heat flux could achieve more pronounced heat transfer enhancement by nanofluids. When the thermal boundary layer developed, the heat transfer enhancement by nanofluids decreased and affected by heat flow become weak. When the mass flow rate increases, the better improvement can be achieved by nanofluids in the micro-channels. The nanoparticles have greater impact on local heat transfer coefficient in thinner thermal boundary layer, leading to the heat transfer enhancement is better at the thermal boundary layer development region in micro-channels.