Mechanism Of Heat Transfer Enhancement Research Articles

Super-long gravity heat pipe geothermal space heating system: A practical case in Taiyuan, China

Using super-long gravity heat pipes (SLGHPs) to exploit deep-earth geothermal energy has indicated its technical superiority and viability; yet there is no practical application so far. The present work reports an SLGHP geothermal space heating system constructed in Taiyuan, China, which has two SLGHPs (2020 m and 2180 m long, respectively) in combination with a single heat pump. The two wells are located only ∼30 m apart, and the well-log temperature at 2000 m depth is around 63 °C for both. Temperatures measured by the optical fiber arranged along the SLGHP outer wall show remarkable uniformity, indicating the good performance of SLGHPs. Testing with the SLGHP system finds that the slightly longer SLGHP extracts 70 % more heat than the other one. Analyses reveal that the heat transfer in the geothermal formations surrounding the shorter SLGHP approximately follows the heat conduction regime while the convection of groundwater contributes less; the high yield of the slightly longer SLGHP is due to an interesting downhole heat transfer enhancement mechanism arising from the interlayer crossflow of groundwater. Further, a numerical model is developed to predict the SLGHP geothermal system's performance in 120-days’ operation. It is found that this system can output 1 MW of heat, sufficing space heating of ∼25,000 m2 buildings. The simulated 20-years’ operation indicates the thermal output degradation is 20.4 % for the shorter SLGHP, whereas it is only 6.8 % for the slightly longer SLGHP; the downhole groundwater crossflow lowers the SLGHP system degradation rate.

Evolution of flow structure and heat transfer enhancement mechanism in impinging jets excited by piezoelectric fan

While jet impingement is recognized for significantly enhancing local convective heat transfer, there remains a considerable challenge in further improving its heat transfer performance and uniformity to meet the growing demand for efficient heat transfer. To address this challenge, this study proposes a method that integrates a piezoelectric fan into a continuous jet, taking advantage of the piezoelectric fan's vibration to deliver pulsating excitation to jet impingement while preserving its continuity. An experimental study is performed to identify the roles of piezo-fan integration on improving convective heat transfer from a continuous circular jet onto a flat target surface. The tests are performed under Rej=2,500∼7,500 (circular jet), s/d = 2∼28 (non-dimensional fan-to-jet distance) and L/d = 2∼8 (non-dimensional jet-to-target distance). In addition, few numerical simulations regarding the flow fields are also made to illustrate the mutual interaction mechanism between piezo-fan and continuous jet. The findings reveal that, in the longitudinal configuration, the average values of heat transfer enhancement factor (χ) and heat transfer uniformity factor (ξ) are approximately 13% and 0.5% higher, respectively, compared to the transverse configuration. In the longitudinal arrangement, the significance of the piezoelectric fan amplifies as the circular nozzle gradually distances itself from the target surface. This fan assumes a pivotal role in augmenting both the uniformity and efficiency of heat transfer associated with the circular jet. Conversely, when the piezoelectric fan moves beyond a specific s/d value (s/d = 10), the interaction between intermittent flow and the wall-jet experiences complete cessation. Despite the existence of a fountain effect resulting from the interaction between the piezoelectric fan and the circular jet in the longitudinal configuration, its discernible impact on overall heat transfer attenuation is minimal. The insights gained from this study will be valuable in improving active strategies for jet impingement and promoting the development of piezoelectric functional devices.

A Study on Boiling Heat Transfer Performance of TiO2-CNTs Hybrid Nanofluid on a Downward-Facing Heating Surface

In-vessel retention through external reactor vessel cooling (IVR-ERVC) is a strategy used to respond to nuclear reactor accidents. One of the key performance indicators determining its feasibility is critical heat flux (CHF). Our focus is on simulating real-world scenarios through surface pool boiling to improve the implementation of the IVR-ERVC strategy with hybrid nanofluids. Two groups of TiO2/COOH-CNTs hybrid nanofluids were prepared: group 1 with different concentrations at the same proportion and group 2 with different proportions at the same total concentration. Researchers compared the improvement of the two groups’ CHF and heat transfer coefficient (HTC), and analyzed the potential mechanism of heat transfer enhancement through roughness of surface, hydrophilicity, and scanning electron microscopy observations. The results showed that a mass concentration of 8 mg:8 mg per liter exhibited the best heat transfer performance, with a CHF enhancement up to 28.21% and an improvement in HTC as well. Meanwhile, correlations between alterations in surface roughness, hydrophilicity, and enhancements in CHF were observed. Finally, by detecting the deposition surface, the possible mechanism of TiO2/COOH-CNTs hybrid nanofluids in enhancing heat transfer was inferred.

Numerical and experimental investigation on boiling heat transfer and flow attributes for the exterior of smooth tubes and T-fin tubes

Experiments and numerical analyses of saturated pool boiling have been carried out on the outer surfaces of horizontal smooth tubes and T-fin tubes. Using a computational model that closely matches the experimental results, the study examined the dynamics of the bubbles as well as the boiling heat transfer coefficients in both the circumferential and axial orientations of the T-fin tubes. For the first time, comprehensive information on the process of high-performance external tube boiling has been made available. The findings demonstrate that an increase in fin density on the surface of the T-fin tubes leads to an abundance of bubble nuclei, with the escaping bubbles exhibiting both increased size and accelerated velocity. Due to the distinctive configuration of the T-fin tube, the local heat flow is lowest at the root fin and highest at the fin tip, indicating that heat transfer is inhibited in the narrow cavity between the two lateral fins. The optimal fin density, which corresponds to the maximum overall heat transfer coefficient has been identified. This states that the enhanced heat transfer mechanism within the T-fin tube is a symbiotic effect of both the increase in heat transfer area and the subsequent degradation of heat transfer fostered by the bubbling of the tube.

Comparative thermo-fluidic analysis of condensation characteristics inside smooth and enhanced tubes

Three-dimensional simulations are performed to investigate the thermo-fluidic characteristics during the flow condensation of representative R134a refrigerant inside smooth and enhanced tubes. Along with a perfectly smooth surface, four different surface structures, i.e., hemispherical ribs, conical fins, axial, and circumferential continuous protrusions (tunnels and huddles) on the inner surface of the tube are tried to understand the heat transfer enhancement mechanism. Simulations are carried out for the mass flux ranging from 100 to 200 kg m−2 s−1 and inlet vapor quality of 0.8 at a phase change temperature of 40 °C. The effect of structures on the flow behavior is analyzed, and the presence of directional condensate drainage near the protrusions is observed. The qualitative and quantitative examinations of interfacial structures at different axial and longitudinal sections are also presented to better understand the distinctive condensation phenomenon for smooth and enhanced tubes. The spatial and time-averaged vapor fraction and liquid film thickness show lower values in the case of enhanced tubes compared to the smooth surface for all tested operating conditions. Furthermore, the hemispherical rib structure showed the highest heat transfer coefficient among the tested structures, whereas a tube with circumferential protrusions (huddles) results in maximum pressure drop during flow condensation. The benefits of heat transfer enhancement appear to be more than the pressure drop penalty for tubes with a conical fin structure and axial tunnels.

Comprehensive investigation of a novel multi-segment heat exchanger for PCM/MF based ocean thermal engine: Numerical analysis, model reduction and experimental work

Phase change material/metal foam based exchangers are widely used in thermal energy storage and ocean engineering. In this paper, a novel heat exchanger called multi-segment heat exchanger is proposed for installation on an ocean thermal engine. This heat exchanger has several concentric circular shaped thin slices inside, which divide the interior into several small areas. During the melting process, independent vortices were formed in each segment. Three aspects of research were conducted: (i) in the investigation of heat transfer enhancement mechanisms, by numerically solving the N-S equations, the reason for enhanced heat transfer in the multi-segment heat exchanger was explained using the field synergy principle theory, and the influence of dimensions and environmental parameters on the melting process was analyzed. Results show that the optimal ratio of height to radius (H/R) for each segment is 1 and the total melting time is reduced by 31–38 %; (ii) for model reduction, by simplifying the full order physical equations for conduction dominant and convection dominant processes separately, ordinary differential equations describing multi-segment heat exchangers were obtained, which can be used for rapid prediction in the engineering practice. The calculation time of the reduced order model is within 0.1 s; (iii) in experimental work, an experimental set-up was built to test the original and the multi-segment heat exchangers, respectively. The numerical work and reduced order model were validated, and results showed that the proposed heat exchanger reduced the melting time by 34 %.

Flow boiling heat transfer performance of R245fa in a vertical enhanced evaporator tube with sintering and electroplating composite coats

The R245fa flow boiling performance was investigated in a vertical evaporator tube of which inner wall surface is coated by a composite sintering and electroplating technology. The effects of vapor quality, heat flux, mass flux, and flow direction were compared and analyzed in the bare tube (BT) and the sintering and electroplating treated tube (SET). The results showed that the heat transfer coefficient increased and then decreased with the rise of vapor quality and heat flux. The heat transfer performance in downward flow was better than that in upward flow, while an opposite trend was observed for frictional pressure drop. With the increased of vapor quality, the frictional pressure drop increased and then decreased, but it monotonically increased with the increasing mass flux. The heat transfer enhancement mechanism was explored through flow pattern visualization. The annular flow pattern conversion line of SET was earlier than that of BT. Due to the strong wettability of SET, the thickness of liquid film was thinner in downward and the mother bubble was bigger in upward, which had a positive effect on flow boiling heat transfer. The maximum the heat transfer enhancement factor (EF) and the performance evaluation parameter (PEC) were 1.83 and 1.75, respectively.

Numerical research of an internal and external-corrugated tube-in-tube helical coil heat exchanger

ABSTRACT To effectively enhance the heat transfer of a tube-in-tube helical coil heat exchanger, a novel internal and external-corrugated tube-in-tube helical coil was proposed by improving both the inner and outer tubes. The impact of the corrugation layout on the annular region’s heat transfer property was simulated. The effect of periodically distributed corrugations on flow patterns and the local Nusselt on the inner tube wall was explored. Furthermore, by comparing the fluid’s velocity and temperature field contour in traditional helical coil and novel helical coil, the enhanced heat transfer mechanism was discovered. Finally, the effects of Re, dimensionless depth H, and dimensionless pitch S of corrugations on the annular region’s heat transfer, flow resistance and entropy generation were analyzed. The results showed that annular region’s heat transfer could be improved due to the periodically distributed corrugations. The local Nu distribution on the corrugated inner tube wall was similar to that of ordinary inner tube, while changed periodically due to the continuous influence of corrugations. The number of Nu and f increased by 53% ~72% and 40% ~120% with the dimensionless corrugation depth and dimensionless pitch being 0.15 and 1.5, respectively. The comprehensive heat transfer performance PEC reached 1.2 ~ 1.6, and cross-arranged inner and outer tube corrugations were more favorable for heat transfer than the corresponding arrangement, when Re = 6000, H3 structure has the highest S t , S f and S gen , which are 0.934W/K, 0.379 W/K and 1.312W/K, respectively. Increasing H and decreasing S could enhance the Nu, f and entropy generation. The values of PEC decreased with increasing S, while with the increment of H, PEC increased and decreased, respectively when Re was in the range of (2000, 4000) and (4000, 6000). In the study range, changing H and S could increase Nu by 40.3%–97.3%, 31.7%–96.8%, and f by 11.7%–176.5%, 16.4%–180.9%, respectively. The final comprehensive heat transfer performance increased by 12.8%-65.9%. Finally, the prediction correlation formulas of Nu and f in NTTHC annular region with high correlation is proposed.

Molecular dynamics study on the effect of surface ionization on the interfacial heat transfer between silica and water

Understanding the thermal transport across the solid–liquid interface is crucial for the advancement of modern technology, with particular emphasis on the silica–water interface due to its universality and remarkable applications in energy conversion and medical treatment. Surface ionization is common for silica surfaces immersed in an aqueous environment, providing an adaptable way to modify silica surface properties. In this study, molecular dynamics simulation was employed to investigate the effect of surface ionization on the interfacial heat transfer between silica and water. It was demonstrated that increasing surface ionization significantly improves interfacial thermal conductance. Interactions between silica and water are strengthened so that more water molecules gather at the interface with higher ionization degree. In addition, increasing ionization results in more hydrogen bonds between ionized silica and water, producing more efficient interfacial heat paths. Somewhat counter-intuitively, although greater surface ionization does result in larger Coulombic forces, the primary contributor to enhanced interfacial thermal conductance is stronger repulsive van der Waals interactions, facilitated by decreased silica–water distances. Overall, this work successfully illustrated the mechanism of interfacial heat transfer enhancement due to surface ionization.

Experimental study on enhanced heat transfer mechanism of U-shaped buried pipe by bio-microbial method

Thermal conductivity of soil is the key factor affecting utilization efficiency of ground source heat pumps (GSHPs). The microbially induced carbonate precipitation (MICP) technology has been proved to be an effective method to improve the thermal conductivity of soil. However, studies on using MICP treated soil as backfill to improve heat transfer performance of GSHP system is still rare. In this study, a small laboratory test device was developed to analyze the effects of different MICP treatment cycles on the heat transfer performance of GSHPs, taking into account four influencing factors: soil temperature, heat transfer and soil ultra-mild soil temperature recovery rate. The results show that MICP treated soil as backfill can improve heat transfer performance of GSHP to a certain extent, and the heat transfer amount in soil are 14.2 %, 34.9 % and 40.1 % corresponding to one to three MICP treatment cycles, which was higher than that of untreated soil. The longer the operation time, the more continuous heat dissipation or heat absorption of the system, the greater the heat transfer radius of the buried pipe temperature. For the influence of number of MICP treatment cycles, heat transfer effect of MICP-treated soil treated three times as backfill is the best. The temperature recovery rate of soil at the central test point of backfill area in summer is 73.15 %, followed by the temperature recovery rate of MICP-treated soil treated twice and treated once, which are 70.95 % and 69.6 % respectively. Considering heat transfer effect, treatment time and treatment cost, secondary MICP treated soil as backfill is the most cost-effective. Through scanning electron microscopy (SEM) image analyses, it is found that calcium carbonate acts as a “thermal bridge” between soil particles, increasing the contact surface between soil particles, resulting in improved heat transfer efficiency among soil particles, and thus improving the thermal conductivity of soil. The results provide good insight for the selection of GSHP layout, operation mode and environmental backfill in engineering practices.

Numerical study on enhanced heat transfer and friction factor characteristics of helical bead-groove tube

The comprehensive thermo-hydraulic performance of a helical tube (HT) is improved in this study by proposing a new type of helical bead-groove tube (HBGT). The heat transfer performance and friction factor of HBGT, helical groove tube, and HT under the same conditions were compared by the numerical simulation method. The effects of different bead-groove positions, bead-groove density ratios, and depth ratios on the HBGT heat transfer and friction factor properties are analyzed. The research introduced the field synergy theory to investigate the enhanced heat transfer mechanism. The results show that the HBGT can strengthen the vortex and secondary flow intensity. Meanwhile, this structure can also reduce the average field synergy angle. Its performance evaluation criterion is increased by a maximum of 8.5% and 28.2% compared to the HBGT and the HT, respectively. Compared with the bead-groove position, the bead-groove density ratio and depth ratio have more significantly influenced the thermal performance of the HBGT. The Nusselt (Nu) and friction factor (f) correlations were calculated from simulation data with error limits of ±5% and ±10%, respectively, to forecast the Nusselt (Nu) and friction factor (f).

Thermodynamic performance study on the novel efficient flow pattern global construction evaporator

Based on the fluid-efficient flow pattern global construction heat transfer enhancement mechanism, a novel evaporator, called the efficient flow pattern global construction evaporator (EFGE), is presented to improve the in-tube evaporation heat transfer at low quality. The numerical analysis and experimental study of EFGE's thermodynamic performance are performed. Results show that the temperature distribution characteristic of the fluid-efficient heat transfer flow pattern construction is better than that of the initial fluid. The evaporation heat transfer coefficient (HTC) of EFGE is 0.34–1.04 times that of a common parallel flow evaporator (PFE), and the pressure drop of EFGE is only 80%–116 % of that of a common PFE at the quality of 0.9. The theoretical nonuniformity of the evaporation HTC between low- and high-quality flow is approximately 12%–67 %, which is 55%–72 % of the pressure drop. The numerical analysis results agree with the finding that EFGE has better thermodynamic performance than PFE in terms of friction power reduction and minimum entropy generation number.

Flow and heat transfer of nanofluids in a cylindrical permeable wavy channel embedded in porous medium using Buongiorno’s model

This investigation deals with the flow of Al2O3–water nanofluid in a wavy porous channel embedded in porous rocks. Fluid exchange takes place uniformly between porous rocks and the wavy channel. In this analysis, cylindrical and parallel plate wavy porous channels are considered. Consequences of Brownian motion and thermophoresis on the flow inside a wavy porous channel are elucidated. The modeled equations are made dimensionless by using dimensionless quantities. Impacts of flow parameters on the velocity, temperature, Nusselt number and friction factor are depicted through graphs. The numerical results with respect to parallel plate wavy porous channel are validated by comparing with the published results. The effectiveness of the cylindrical wavy porous channel as a heat transfer enhancement device in comparison to the parallel plate wavy porous channel is confirmed in this study. Designing devices at microlevels and understanding the heat transfer enhancement mechanism in wavy channels using the nanoparticle addition are the major outcomes from the results of this numerical investigation. The heat and mass transfer rate enhancements in the wavy porous channels are due to the higher Brownian motion in the boundary layer region and accelerated thermophoresis through thermal and concentration boundary layer thicknesses.

Study on the efficient heat transfer mechanism of microchannel pin-fin arrays under low pumping power

The efficient heat transfer with low pressure loss has long been a critical challenge in the microchannel heat sinks (MCHS) design. Further study is required for the quantitative analysis of the longitudinal vortex heat transfer enhancement mechanism. In this research, a microchannel heat transfer experiment and corresponding simulations are conducted. Based on the common Reynolds number range in engineering applications of MCHS, the underlying causes of heat transfer enhancement and pressure loss increase in MCHS with respect to Reynolds numbers is clarified. The results show that when the Reynolds number is 400, the cylindrical wake reduces the synergistic angle between the velocity and the temperature gradient at the streamwise gap, which greatly enhances the heat transfer performance of MCHS. In addition, when the Reynolds number is 900, the early separation of the cylindrical wake vortex leads to a significant reflux near the center of the spanwise gap, which significantly increases the pressure loss. A Different Reynolds number Efficiency Evaluation Criterion (DEEC) is proposed for evaluating the performance of MCHS. It is found that the flow state of cylindrical wake with Reynolds number of 400 has the best heat transfer efficiency. The concept of introducing longitudinal vortices into the pin–fin array can serve as a key guideline in the heat transfer enhancement optimization design of MCHS.

Micro-channel topology optimization based on enhanced heat transfer mechanism

Topology optimization modifies the material distribution in the design domain to produce micro-channel structure with improved thermal performance. In this work, five heat dissipation micro-channel structures with various design domain aspect ratios are optimally designed based on the bi-objective topology optimization method. The optimal design variable fields, temperature fields, and pressure fields are subsequently obtained for each operating condition, and the flow heat transfer effect and the enhanced heat transfer mechanism are investigated under various working conditions. On this basis, the flow heat transfer impact of micro-channels under various operating situations is optimized and studied by combining the field synergy concept and entransy dissipation theory. The findings show that when the Reynolds number rises in the laminar flow region, the complexity of the topological flow channels also rises. The average temperature, Tave,decreases, Nusselt number rises, the inlet and outlet pressure drop, ?P, gradually increases, the integrated enhanced heat transfer factor PEC gradually decreases, the field synergy number, Fc, increases, the pressure drop synergy angle, ?, gradually increases, the entransy dissipation, Evh, increases, and the flow heat transfer performance of each heat dissipation channel is also enhanced due to the complex channels and high Reynolds number in the domain. The investigation of micro-channels with various topologies revealed that the micro-channels with the same boundary conditions and a design domain aspect ratio of 25/64 had the best synergy effects of velocity-pressure gradient and velocity-temperature gradient, the best heat transfer effect, and the best flow characteristics.

Heat transfer enhancement mechanism of elliptical cylinder for minichannels with delta winglet longitudinal vortex generators

In order to improve the heat transfer performance in the minichannel with delta winglet longitudinal vortex generators and obtain the heat transfer enhancement mechanism, the flow and heat transfer of four basic models (CFD-I, CFD-V, CFU-I and CFU-V configurations) and four enhanced models with elliptical cylinders are numerically simulated in the range of Reynolds number from 2500 to 10,000. The friction factor, Nusselt number and thermal performance factor are evaluated, and the secondary flow intensity and entropy generation rate are analyzed. The main conclusions are as follows: compared with the basic models, the Nusselt number increase by 8.4–14.1 %, 6.1–12.0 %, −1.9-5.0 % and 11.0–14.9 % respectively after adding the elliptical cylinders, and the thermal performance factor of these four models with elliptical cylinders are within 1.05–1.25, 1.00–1.13, 0.97–1.16 and 1.03–1.22, respectively. Compared with the smooth channel, the Nusselt number of CFD-V and CFU-I configurations with elliptical cylinders increased by up to 61 % and 63 %, respectively, and the overall thermal performance increased by up to 25 % and 22 %, respectively. These two configurations are recommended because they make full use of the fluid accelerated by the elliptical cylinder and effectively improve the strength and influence range of the longitudinal vortexes.

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.

Flow and heat transfer characteristics in adaptive latticework structure designed using shape memory alloys

In this study, numerical simulation and particle image velocimetry experiments were performed to investigate an adaptive flow and heat transfer control structure based on shape memory alloy operating in the range of Reynolds number 100 to 2000. The adaptive structure could operates with low drag coefficient in the flat state, and actively enhance heat transfer with a latticework channel structure in the warping state. Different warping heights and Reynolds numbers were investigated to analyze the flow patterns and heat transfer enhancement mechanism. The warping of the shape memory alloy leads to flow deflection and generates a z-direction velocity component around the gap, which is the main reason for local heat transfer enhancement. The average velocity in the main stream plane (XY plane) close to the heat transfer surface increases sharply with warping. This study found significant increase in normal velocity and in-plane velocity (up to 0.17 and 0.94 times reference inlet velocity uin) in the plane at a distance of 0.1 times hydraulic diameter Dh from the wall due to warping, which strengthens the disturbance near the wall and thus enhances heat transfer. The longitude vortices generated in the structure are divided into the main vortex in the center of the channel and the corner vortex in the sub channel. The corner vortex is mainly caused by the gap flow between the shape memory alloy and the wall surface. In the warping state, the average value of the main stream velocity components near the wall is relatively close to the numerical simulation results (by deviation of 0.04 uin). In numerical results, the structure can achieve a heat transfer regulation ratio of 2.5–3.3 times and a resistance regulation ratio of 5.2–14.6. The performance parameters are not monotonous with the increase of the warping height or the Reynolds number. When the warping height is 0.6 Dh and Reynolds number equals to 400, maximum performance factor (1.34) is achieved.

Venturi effect-based simulation of air-based PVT optimization

ABSTRACT Air-based photovoltaic thermal(PVT) solar collectors can convert solar energy into both electrical and thermal energy. However, the output efficiency was often low due to the limitation of the heat transfer capacity for air. Therefore, in this paper, an air-based PVT with the application of a novel rib was modeled and numerically simulated. The innovation of this paper was proposing a novel rib, which was based on the Venturi effect. First, the effects of the new rib control parameters on the performance of the PVT system were analyzed using orthogonal design tests. The result showed that the thermal efficiency of the best factors combination test was 51.16% when the solar radiation was 500 W / m 2 and the air inlet rate was 0.09 m / s . Second, analyzing the fluid flow characteristics and the enhanced heat transfer mechanism, this study found that the increased air velocity under the novel rib and the low-velocity vortex formed between the rib both negatively affected heat transfer. Consequently, this paper proposed a novel fin design coupled with curved fins to address the issues with the novel rib design, which achieved 55.12% thermal efficiency with the combination structure. Finally, we applied the combined structure to the PVT system, studied its thermal efficiency under real-world conditions, and compared it with the empty channel collector. The results showed significant improvement, with a 16.38% increase in thermal efficiency and the model exhibited excellent, stable performance.

Experimental study on combined heat transfer enhancement due to macro-structured surface and electric field during electrospray cooling

Electrospray (ES) cooling is promising to achieve the high-heat-flux thermal dissipation with a low fluid consumption when using structured surfaces as heat sinks of micro-size and high-power electronics. Nevertheless, available research is still limited in the fundamental of combined heat transfer enhancement owing to the macro-structured surface and electric field. In this study, four macro-structured surfaces machined with cubic pin fins, conical pin fins, wavy fins and straight fins are compared to one flat surface under electric field to shed lights on the combined heat transfer enhancement mechanism during ethanol ES cooling. The experimental results demonstrate that both electric field and surface modification can elevate the critical heat flux (CHF) and heat transfer coefficient. A CHF of 7.93 W/cm2 is achieved by the cubic-pin-fin surface with an applied voltage of 4.5 kV at Ts = 145 °C, corresponding to a 63 % enhancement over the flat surface. The CHFs at first increase and then decline with the increased area ratio (f), and the superior heat transfer performance is achieved when f = 0.79. In the low temperature region (30 °C < Ts < 51 °C), the enhanced convection and bubble formations caused by electric field are deemed as the dominated heat transfer mechanism. The increase in nucleation sites triggered by macro-structured surfaces governs the heat transfer in the mid-temperature region (51 °C < Ts < 119 °C). Whereas, the heat transfer enhancement due to macro-structured surfaces declines in the high temperature region (Ts > 119 °C), possibly as a consequence of the large thermal resistance induced by the Leidenfrost phenomenon.