Equivalent Heat Transfer Coefficient Research Articles

Multi-Aspect Shaping of the Building’s Heat Balance

In the European Union, buildings account for 42% of the energy consumption and 36% of the direct and indirect energy-related greenhouse gas emissions. Reducing thermal power for heating purposes is crucial to achieve climate neutrality. The main purpose of this article is to identify the places in the building where it is possible to significantly improve energy efficiency through the use of appropriate construction and material solutions. This article contains a multi-aspect approach to the heat balance of a building. Solutions that have a direct impact on building energy consumption were analysed, taking into account architectural, technological, and material aspects. Particular attention was paid to energy-efficient design and material solutions for non-transparent and transparent external walls and thermal storage walls (Trombe walls). An analysis of heat transfer through building elements was carried out, along with the optimisation of energy-efficient solutions for non-transparent and transparent barriers. Two methods for determining the equivalent heat transfer coefficient Ue for solar active partitions are presented. The analysis presented in the work using the original method of the balanced heat transfer coefficient Ue is a testing ground for identifying unfavourable features of the building structure, as well as the most energy-efficient solutions that can be used in establishing standards for the construction and modernisation of buildings. The value of the Ue coefficient illustrates the actual heat transfer through the partition. Having Ue values for various structural solutions of building envelopes, the designer can easily select the most effective ones. The use of the presented methodology will allow for the optimisation of technical solutions for building elements to improve its energy efficiency.

Sustainable off-grid residential heating and desalination: Integration of biomass boiler and solar energy with environmental impact analysis

As global energy demand rises and the environmental impact of fossil fuel-based heating systems becomes increasingly evident, transitioning to renewable energy sources like solar and biomass for residential use is imperative. This is further underscored by the growing scarcity of freshwater resources, especially in arid urban environments. This study introduces an innovative off-grid system that effectively integrates a floor heating system with a vapor compression desalination unit. The novelty of this study lies in its approach to combining solar and biomass energies for both heating and desalination in a single off-grid system. The system's electrical and thermal energy needs are met by combining photovoltaic thermal collectors and a biomass boiler. Using TRNSYS software, the system's performance was evaluated in varying design configurations across Riyadh, Doha, and Abu Dhabi, targeting optimal thermal comfort for a standard residence with four occupants. Results show Riyadh leading in solar fraction at 48.43%, followed by Abu Dhabi (47.23%) and Doha (44.85%). The optimal equivalent heat transfer coefficient for the floor heating system varied notably between these cities, with Abu Dhabi requiring a lower coefficient to achieve thermal comfort. Additionally, a life cycle analysis revealed that the biomass boiler is the predominant source of CO2 emissions, contributing over 70% of the total in each city. Riyadh exhibited the lowest CO2 emissions, attributed to its higher solar fraction. This study demonstrates the system's versatility across different urban climates and its potential to lessen dependence on traditional energy sources, positioning it as a promising solution for future off-grid applications.

A finite element model with multi-flow fields for the quenching process of mill liner component made of bainite and martensite: simulation and experimental validation

Controlling the type and location of microstructure transformation through heat treatment processes is the key to improving the property of workpieces, especially they are large-sized and highly thick irregular. Such as mill liner, the worst wear in mining machinery, of which microstructure inhomogeneity is key to developing cost-efficient mining process. In order for advanced high strength steel (AHSS) to be used in mining machinery, its microstructure uniformity must be optimized, usually by adjusting the process parameters of the heat treatment process to optimize the microstructure distribution uniformity. However, the research on the simulation of large-sized and highly thick irregular heat treatment process is limited. Previous studies usually controlled the heat transfer process by using the heat transfer coefficient as the boundary condition, but only applied to small size and simple structure of the workpiece. As the heat transfer coefficient is a process quantity, its size depends on the thermal property parameters of the fluid on both sides of the wall, the flow rate, the shape of the solid surface, the thermal property number of the material and other factors, which is not suitable for large-sized and highly thick irregular. Therefore, for the large-sized and highly thick irregular heat treatment simulation process, the equivalent heat transfer coefficient of flow field was introduced as the boundary condition, and the temperature field, microstructure field and stress field were coupled to improve the accuracy of the numerical model. The accuracy of the finite element model is verified by comparing the temperature simulation error of less than 10% through temperature measurement experiments of the quenched workpiece. The evolution process of heat transfer in large-sized and highly thick irregular water bath quenching is obtained through the finite element model data. The difference in the flow rate of the wall fluid is the fundamental reason for the difference in cooling effect, and the wetting process of the wall is the secondary reason. At the same time, the corresponding microstructure and surface hardness distribution of different cooling regions are obtained, which can provide more accurate guidance for the optimization of heat treatment process of large size and thick special-shaped parts.

Thermal performance investigation of membrane-assisted radiant cooling system for localised outdoor cooling hub

Heat waves pose increasing threats to human well-being, especially in urban areas, and staying safe beyond comfort during heat waves is a crucial issue. Localised cooling hubs may provide thermal stress relief in the outdoor environment. For outdoor cooling applications, conventional air conditioning systems are not effective but energy-intensive, and the membrane-assisted radiant cooling system can be a potential alternative, which directly treats the radiant loads while minimising the convective heat loss to the ambient. In the present study, a numerical model is developed to analyse the heat transfer behavior of membrane-assisted radiant cooling for outdoor applications, considering the effect of solar and long-wave radiation and wind parameters. As the transmissivity of the membrane material plays a crucial role in reducing the amount of solar heat, the performances of three types of membranes, i.e., non-selective, mid-infrared selective, and sky-window selective, were compared and analysed at different times on a typical summer day in Hong Kong. The results indicate that the sky-window selective membrane, which allows only transmission of infrared in the 8–15 µm wavelength range, saves energy up to 44 % and 47% compared with mid-infrared selective and non-selective membranes, respectively. In the outdoor application, the equivalent convective heat transfer coefficient of the cooling panel can be reduced in the range of 2.2 W/m2K to 2.6 W/m2K. It is observed that, when the panel is not exposed to direct solar radiation, there is no advantage in using the mid-infrared selective membrane over the non-selective membrane. However, under direct solar radiation, a non-selective membrane-assisted cooling panel demonstrates poor performance, absorbing solar heat flux 2.27 times higher compared to MIR-selective membrane-assisted panel.

Experimental study on the thermal performance of building external window greenery in a subtropical climate

Previous research has found that building greenery systems can cool both buildings and their surrounding environments during the summer. However, the window greenery system, which is a widely adopted form of building greenery, has received limited attention. This study analyzed the thermal performance of window greenery systems in general and the building-integrated window gardens in specific, based on in-situ measurements in a subtropical city. The results show that the cooling effects of the window garden on the envelope were significant near the bottom region. During the air-conditioning period, the heat flux through the bottom wall and windowpane interior surfaces was decreased by 29.0 % and 25.8 %, respectively. A regression model for heat flux through the bottom windowpane was proposed, which estimated that the reduction in radiative heat transfer by plants was 88.0 %. The equivalent heat transfer coefficient for the walls adjacent to the window garden was calculated. Window gardens also improved the outdoor microclimate, reduced the average air temperature over planting troughs by 1.0 °C, and increased the average relative air humidity by around 10.0 % throughout the day. Given that window gardens’ cooling effects strongly depend on water evaporation, a win–win strategy for energy and water saving by harvesting the air conditioning condensate for irrigation is proposed. Based on an estimation, the condensate could fully meet the daily requirements of window greenery, confirming the feasibility of the win–win strategy.

On the design of a hydrogen micro-rectangular combustor for portable thermoelectric generators

Micro-combustion based thermoelectric generators have been proposed to power micro-electromechanical units. The present study aimed to optimize the thermal design of micro-combustor to best suit the requirement of thermoelectric generators. An equivalent heat transfer coefficient was used to consider the arranged thermoelectric generators and its effect was elucidated. Increasing the coefficient enhances combustor wall heat loss, reducing flame stability and uniformity index of temperature. It is thus impossible to constrain combustor outer wall temperature below the maximum allowable temperature of commercial thermoelectric generators (623K) simply through increasing equivalent heat transfer coefficient. To address this issue, three simple strategies of combustor channel modification, adding copper layer and using flue gas heat recirculation were developed. A notable improvement in uniformity index of temperature over 6.5% is achieved for the latter two strategies. Nevertheless, none of these three strategies can ensure outer wall temperature below 623K. Motivated by the different mechanisms to improve temperature uniformity and reduce wall temperature for the two simple strategies of adding copper layer and using flue gas heat recirculation, a hybrid strategy combining them was proposed. The hybrid strategy can maintain flame stability, improve uniformity index of temperature approach to 100% and control combustor outer wall temperature below 623K, making it a qualified strategy for designing high-performance micro-combustors.

A simplified method for the thermal performance analysis of double-layer pipe-embedded external wall

Reducing the heat dissipation through the envelope is critical for heating energy savings in buildings. Double-layer pipe-embedded external wall (DPEW) has the advantage of load interception and low-temperature heating, and is a promising solution to reducing energy demand for heating. To gain a comprehensive understanding of the thermal performance of DPEW, a thermal network model is established in this study, which characterizes the steady heat transfer process of DPEW by the superposition of water temperature, outdoor temperature and indoor temperature. The accuracy of the proposed model is validated against experiment data in the literature. Evaluation indexes including equivalent overall heat transfer coefficient (U-value), equivalent outdoor temperature, and energy utilization rate of outer pipe or/and inner pipe are proposed based on the thermal network model of DPEW. A case study is conducted under typical winter conditions of Beijing to investigate the influence of water temperature and outdoor temperature on the thermal performance of DPEW. The results show that the energy utilization rate of DPEW increases with the increasing outdoor temperature and inner pipe water temperature, and the decreasing outer pipe water temperature, while the energy utilization rates of the outer pipe and inner pipe remain largely unaffected. At the outdoor temperature of −9 °C, the outer pipe water temperature of 10 °C and the inner pipe water temperature of 25 °C, the energy utilization rates of the outer pipe, inner pipe, and DPEW reach approximately 27 %, 89 % and 65 %, respectively. This study gives an insight into the steady-state thermal performance of DPEW, and provides guidance for its further research and application.

A simplified cooling load calculation method based on equivalent heat transfer coefficient for large space buildings with a stratified air-conditioning system

A simplified cooling load calculation method based on equivalent heat transfer coefficient for large space buildings with a stratified air-conditioning system

Analysis of Fluid-Solid Coupling Radial Heat Transfer Characteristics in a Normal Hexagonal Bundle Regenerator under Oscillating Flow

The main purpose of this paper is to analyze the heat transfer mechanism of a new type of regenerator with a low temperature difference and low current resistance under oscillatory flow at room temperature. Taking the single tube of the regenerator as the research object, the exact analytical solution of the radial heat transfer characteristics of the regenerator is obtained by studying its analytical model. The factors affecting the heat transfer characteristics are analyzed, and then the regenerator is optimized to improve the performance and efficiency of the regenerator system. In this study, we systematically analyzed the radial heat transfer characteristics of a regenerator under isochoric process conditions. A closed-system physical model of the incompressible isochoric process under oscillating flow was established. Then, the radial analytical solutions of pressure fluctuation, fluid velocity, fluid-solid temperature, and heat were derived in the complex number field. Furthermore, the fluid velocity, fluid-solid coupling wall temperature, heat, and equivalent heat transfer coefficient were assessed. Furthermore, the influences of frequency, inner diameter R1 of the regenerator, and different working medium and materials on the above parameters were discussed. It was found that the analysis and evaluation of fluid velocity, fluid-structure coupling wall temperature, heat, and equivalent heat transfer coefficient are helpful in understanding the dynamic characteristics of radial heat transfer in a regenerator system. Through the study of radial heat transfer under oscillating flow, it was found that the working medium, frequency, inner diameter of the regenerator, and material quality of the regenerator are helpful for the design optimization of the regenerator. Furthermore, our investigations established that the variation law of wall fluid-solid coupling temperature amplitude could be divided into three parts: the unidirectional flow part; the low-frequency part, where the temperature amplitude falls rapidly with increasing frequency; and the high-frequency part, where the temperature amplitude increases with the frequency. In addition, the variation of radial heat transfer of the fluid-solid coupling surface is similar to the changes in the temperature amplitude. We also discovered that the equivalent heat transfer coefficient of the fluid-solid surface is related to thermal conductivity of the material. Specifically, larger thermal conductivity values result in greater equivalent heat transfer coefficients. Based on the research into the radial heat transfer characteristics, the new regenerator has great application potential in the Stirling air conditioning system at room temperature.

Experimental and Theoretical Study on Operation Characteristics of an Oscillating Heat Pipe

An oscillating heat pipe (OHP) is an effective heat transfer device for the thermal management of electronic devices. However, the heat transfer mechanism of the OHP was not fully understood due to its complicated operation characteristics. In this paper, the thermal performance of an OHP was experimentally studied. The condensation and evaporation temperature variations were monitored under different heat inputs and were then used to evaluate the OHP system operating characteristics. Thermal resistance was used as a key parameter to evaluate the thermal performance of the OHP system. The results indicated that as the heat input increased from 25 to 100 W, the average thermal resistance decreased while the stable evaporating and condensing temperatures increased. The equivalent heat transfer coefficient was derived theoretically. It showed that the reciprocal of the radial heat transfer coefficient increased with increasing liquid film thickness. Based on this result, an empirical correlation was proposed to evaluate the thermal resistance of an OHP system. This correlation was validated using both the experimental data provided in this study and the data collected from the open literature. The comparison results indicated that the proposed empirical correlation could reasonably predict the thermal resistance under different filling ratios and heat inputs.

Model Establishment of Chip Air Cooling Process and Its Proportional Integral Differential Tuning

Abstract Chip junction temperature is a key factor affecting the normal operation of the chip. The development of integrated circuit technology brings about high integration and low cost, but it also puts forward higher requirements for the cooling system. This paper focuses on the air cooling of the chip, builds a hardware test platform based on MCS-52, the general name of the intel series microcontroller unit, and sets up a mathematical model of the air cooling process of the chip on the matlab platform based on the principle of energy conservation, heat transfer theory and finite element method. By proposing the equivalent convective heat transfer coefficient, the thermal resistance of the system can be well estimated. This model can easily realize the joint simulation of chip, heat radiator, and control strategy, which overcomes the disadvantage that traditional finite element simulation software are difficult to combine with control strategy. In addition, based on the model, the proportional integral differential (PID) control parameters are automatically optimized, achieving excellent temperature control effect, and proving the feasibility of optimizing the control parameters through the model.

Experimental Study on the Thermal Performance of Flat Loop Heat Pipe Applied in Data Center Cooling

The cooling system is the auxiliary equipment that consumes the most energy in a data center, accounting for about 30 to 50% of the total energy consumption. In order to effectively reduce the energy consumption of a data center, it is very important to improve the heat exchange efficiency at the chip level. Compared with air cooling, single-phase cold plate liquid cooling, and immersion liquid cooling, the flat loop heat pipe (FLHP) is considered to be a better chip-level cooling solution for data centers. It has extremely high heat transfer efficiency and heat flux variability, and it can avoid the operation risk caused by liquid entering the server. In this paper, a FLHP with an evaporator designed with a “Tesla valve” flow channel configuration is developed. Experiments on the FLHP are carried out, focusing on the installation angles and cooling condition factors. The results show that an inclination angle of 20° is the critical point of the influence of gravity on the performance of the FLHP; to ensure good operation of the FLHP, the installation angle should be greater than 20°. The equivalent heat transfer coefficients of the FLHP condenser under different cooling conditions are calculated. It is found that water cooling can provide higher cooling heat transfer coefficients with lower energy consumption and operating noise. Additionally, the heat transfer limit, operating temperature uniformity, and start-up stability of the FLHP are significantly improved under water cooling conditions. The maximum heat load of the FLHP is up to 230 W, and the temperature difference of the evaporator surface can be controlled within 0.5 °C, under 20 °C water cooling. Finally, using the FLHP for thermal management of the chip, its heat transfer efficiency is 166 and 41% higher than that of air cooling and water cooling, respectively.

Optimal Design of Wearable Micro Thermoelectric Generator Working in a Height-Confined Space

With the increasing development of self-powered wearable electronic devices, there is a growing interest in thermoelectric generators (TEGs). To achieve more comprehensive and reliable functionality of wearable devices, improving the power generation performance of thermoelectric devices will be the key. It has been shown that integrating a heat sink at the cold end of the TEG increases the effective temperature difference and, thus, maximizes the power output of the thermoelectric device. However, the space left for the power supply is often limited. How to optimize the integrated system of micro-thermoelectric generators and heat sinks in a height-confined space has become the key. In this study, we have established a corresponding model using a numerical calculation method, systematically studied the influence of TEG geometric size on the number of fins and fin height, and determined the optimal number of fins for the highest equivalent convective heat transfer coefficient corresponding to different fin heights. We also conducted the co-design of TEG and fin topological structure to study the effects of the ratio of leg height to fin height (l/H), the width of legs (w), and the number of thermoelectric leg pairs (N) on the maximum output power density per unit area (Pm1) and the maximum output power density per unit mass (Pm2) of the device. When N = 16, w = 0.3 mm, l/H = 2.5 (that is, l = 3.57 mm, H = 1.43 mm), and Pm1 reaches the maximum value of 30.5 μW/cm2; When N = 2, l/H = 0.25 and w = 0.3 mm, and Pm2 reaches a maximum value of 5.12 mW/g. The measured values of the open-circuit voltages of fabricated micro-TEGs with different thermoelectric leg heights (l = 0.49 mm, l = 1.38 mm, and l = 1.88 mm) are basically consistent with the simulated values. When N = 2, l = 0.49 mm, H = 3.74 mm, and w = 0.85 mm, and Pm2 reaches 0.44 mW/g. The results provide insights into the optimal design of wearable micro thermoelectric generator working in a height-confined space and highlight the importance of co-designing TEGs and fin topological structures for optimizing their performance.

Equivalent Convective Heat Transfer Coefficient for Boundary Conditions in Temperature Prediction of Early-Age Concrete Elements Using FD and PSO

Equivalent Convective Heat Transfer Coefficient for Boundary Conditions in Temperature Prediction of Early-Age Concrete Elements Using FD and PSO

Thermal Modeling and Analysis of Hybrid-Magnetic-Circuit Variable Flux Memory Machine

Variable flux memory machine (VFMM) can be regarded as a promising candidate for electric vehicles (EVs) owing to its advantages of excellent flux adjustment capability and overall efficiency improvement over an extended operating range. However, the uncertain internal thermal behavior due to magnetization state (MS) change has an immense influence on electromagnetic performance of permanent magnet, which makes the precise MS estimation of low coercive force PM a challenging issue. To explore the benefits of MS manipulation on temperature rise reduction and provide an effective method to accurately evaluate temperature rise in VFMM, a magnetic-thermal coupling model of the newly developed hybrid-magnetic-circuit VFMM accounting for different MSs is established in this article. The lumped parameter thermal network (LPTN) model and computational fluid dynamics based finite-element (FE) method are employed to analyze the temperature characteristics, respectively. First, the losses of the investigated machine are calculated by adopting both the FE method and analytical equations. Afterward, the equivalent thermal resistances and heat transfer coefficients are determined by employing the LPTN model. Meanwhile, FE modeling is employed to analyze the three-dimensional thermal field distribution. Finally, a prototype machine is fabricated and experimentally measured, which can confirm the validity of the theoretical analyses.

A thermal environment prediction method for a mine ventilation roadway based on a numerical method: A case study

Thermal environment prediction has become increasingly significant in recent years with its promise of wide application in underground structures. The current work presents a numerical method for the feasible and effective analysis of the airflow and surrounding rock temperatures in ultralong mine ventilation roadways to understand their dynamic heat transfer. To reduce the modeling effort and computation time with satisfactory accuracy, the ventilation roadway is approximated as a one-dimensional (1D) line element. Still, the surrounding rock of the roadway remains three-dimensional (3D). An equivalent heat transfer coefficient calculates the dynamic heat transfer in the radial direction of the roadway. A case analysis of the Sanhejian coal mine ventilation roadways in China is performed, and a comparison between the simulated results and field measurements indicates that the predicted airflow temperature shows good agreement. The change in the underground thermal environment concerning ventilation time is systematically investigated in detail. The surrounding rock of the roadway adjusts the underground thermal environment through heat absorption or heat release, and the temperature distribution of the surrounding rock presents a "V" shape in winter and a "W" shape in summer. The self-compression of air contributes to a remarkable heat source for the increase in airflow temperature in the air intake shaft. A roadway with a low initial temperature of the surrounding rock after long-term ventilation can effectively cool the high-temperature airflow underground in early summer. The reduction in ventilation volume will increase the cooling degree of airflow by the surrounding rock in long-term ventilation roadways in summer. Still, the airflow will be heated more significantly in short-term ventilation roadways. In addition, the increase in the average annual ambient temperature will result in a linear rise in underground airflow temperature.

Heat transfer enhancement by diamond nanofluid in gravity heat pipe for waste heat recovery

Waste heat recovery is significant for improving energy utilization, reducing carbon emissions, and neutrality. The gravity heat pipe (GHP) has excellent thermal performance due to the cyclic phase transformation of the working fluid. As an important thermal management device for waste heat recovery, the heat transport capacity of GHP improves, the efficiency and performance of the waste heat recovery increase, and more wasted heat can be stored more quickly. Nano-diamond has the highest thermal conductivity and can be dispersed in water to form a diamond nanofluid, enhancing the thermal performance of GHP. In contrast, the study on the heat transfer behavior of the diamond nanofluid in GHP is insufficient. Besides, the influences of filling ratio (FR), mass fraction (MF), and heat flux on thermal performance are in demand for further study. In this article, the heat transfer behavior is investigated by studying the flow patterns of diamond nanofluids. The influences of filling ratio and mass fraction on flow patterns are analyzed. An orthogonal experiment is conducted; the heat flux has the most significant effect on the thermal performance, followed by the filling ratio and mass fraction. The thermal performance is the best when the optimal parameters (FR = 20%, MF = 1 w.t.%) are selected under a heat flux of 20 × 104 W/m2. The equivalent heat transfer coefficient reaches 3485 W/(m2·°C). This article can achieve a deeper understanding of the diamond nanofluid heat transfer mechanism in GHP and enhance the thermal performance of GHP for better waste heat recovery.

Experimental study on the thermal performances of a non-pump-driven thermo-activated building system based on flat-plate heat pipe array

• A non-pump-driven thermo-activated building system based on FPHPs is proposed. • Effects of tilt angles and heat loads on FPHP are experimentally investigated. • Preliminary performances of the FPHP-embedded wall are experimentally investigated. • The optimal tilt angle of FPHP is not a fixed value for building integration. • The measured U d is 0.19 and −0.08 W/m 2 °C when Ths is 25 and 35 °C, respectively. As a burgeoning active thermal insulation technique, the pipe-embedded building system has attracted particular attention from architects and engineers. A novel pipe-embedded building system based on flat-plate heat pipe (FPHP) is proposed to solve the inherent issues like operation energy consumption and façade integration. The experimental investigation is firstly conducted on an FPHP prototype to explore its thermal response characteristics at different tilt angles and heat loads and then performed on an FPHP-embedded composite wall to explore the preliminary thermal performances. Results showed that the length of the condensation section with uniform temperature distribution accounted for at least 88.9% when the heat load was smaller than 3000 W/m 2 , which verified the feasibility of using FPHP to complete the heat charging process. The tilt angle had a significant influence on the thermal resistances, startup behavior, and effective thermal conductivity of FPHP, and the optimal tilt angle could be reduced for load reduction mode, while this value could be increased for supplementary heating mode. When the heat source temperature is 25 and 35 °C, the equivalent heat transfer coefficient value of the FPHP-embedded wall under test conditions is 0.19 and −0.08 W/m 2 ·°C, respectively. Meanwhile, the corresponding heat loss reduction rate could reach 62.7% and 116.4%, respectively. Overall, the results evidenced the effectiveness of FPHP-embedded building systems, which could also contribute to its system design, operation control, and further investigation.

Analytical Solution of Heat Transfer Performance of Grid Regenerator in Inverse Stirling Cycle

The regenerator plays an extremely important role in the Stirling circulation. A grid regenerator can be used for inverse Stirling machines at room temperature due to its low flow resistance. This paper proposes a hexagonal grid regenerator to theoretically explore heat transfer properties in the inverse Stirling cycle and establishes an approximate analytical model to analyze the effect mechanism of working frequency, thermal diffusivity and wall thickness on the oscillation flow. The results show that the wall thickness is one of the key factors affecting the equivalent heat transfer coefficient. Specifically, too small or too large wall thickness increases the instability of the heat transfer process. The ultimate wall thickness is determined by the equivalent heat transfer coefficient and thermal penetration depth, whose optimal value ensures not only sufficient heat exchange but also the full utilization of materials. With the increase in frequency, heat exchange performance is improved monotonously. Therefore, high–frequency operation can improve the heat exchange performance of the regenerator. In addition, an optimization criterion for the size of regenerator with the specific capacity of heat transfer as the objective parameter is proposed based on the equivalent heat transfer coefficient. The optimal parameters were obtained when relative thickness was set as 0.8 mm and the equivalent heat transfer coefficient was up to 104–105 W/m2·K indicating that the grid regenerator has broad application prospects in the inverse Stirling cycle.

Experimental study on synergistic enhancement of thermal performance of a closed two-phase thermosyphon by a TiO2 nanofluid doped with Al2O3

The heat transfer performance of two-phase closed thermosyphon solar collectors (TPCTs) can be improved by adding nanoparticles to the conventional working fluid, but it is necessary to ensure good rheological properties while improving the heat transfer performance. Therefore, spherical (TiO2) nanoparticles with good rheological properties and rod-shaped (Al2O3) nanoparticles with high thermal conductivity were mixed and applied to the TPCT. First, the effects of individual factors such as the concentration of nanoparticles and the operating parameters on the thermal performance of the TPCT were investigated and a regression analysis was performed. Then, the joint effect of inclination angle and filling ratio on the thermal performance of TPCT was further investigated by orthogonal experiment. The results show that doping TiO2 with Al2O3 can further improve the thermal performance of TPCT. For the volume concentration of 0.6% of nanoparticles, filling ratio of 50% and inclination angle of 80°, the best heat transfer performance of TPCT was achieved. Compared with the deionized water (DW), the thermal resistance of the single (TiO2–H2O) and hybrid (Al2O3+TiO2–H2O) nanofluid TPCT is reduced by 10.7% and 13.4%, the thermal efficiency is increased by 11.0% and 13.7%, and the equivalent heat transfer coefficient is increased by 16.2% and 18.7%, respectively.