查看更多>>摘要：The recently developed composite porous vapor chamber (CPVC) with central and radial grooves shows small thermal resistance and good thermal performance under high heat fluxes. However, the groove parameters are difficult to optimize due to their coupling relationship and the lack of sound scientific ground. This paper addresses the issues by proposing an optimization method based on design of experiments (DOE) and response surface methodology (RSM) with the help of a developed decoupling strategy. In this method, a decoupling strategy inspired by the golden section rule is proposed to break the coupling relationship between groove parameters. Then DOE and RSM analyses are performed to provide data for single-objective and multi-objective optimization of groove parameters. Results indicate that, due to the decoupling strategy, the total number of sampling points is reduced by about 40% and the ratio of effective sampling points is increased by about 65.43% in DOE analysis. The influence of the groove parameters is nonmonotonic, and groove depth (T) has the greatest effect on the thermal hydraulic characteristics of CPVC, while the influence of central circular diameter (D) and groove width (W) is similar. The effect trend of the number of grooves (N) is contrary to that of W at small values due to the dramatic decrease of W. The thermal hydraulic performances can not reach the minimum or maximum value simultaneously, and the groove parameter configuration of D = 29 mm, W = 3.7 mm, T = 2.1 mm and N = 16 can be chosen as the optimal, leading to the decrease of the maximum temperature and the maximum temperature difference on the condensing surface, and the liquid pressure drop by 0.005 K, 0.015 K and 9.68%, respectively, compared with the initial design. The proposed method may pave a more objective and scientific alternative to design the structural parameters of CPVC.
查看更多>>摘要：This article deals with the possibility of using PCMs in cooling ceiling systems to reduce significantly air temperature fluctuations and energy demands for cooling. The PCM-based cooling ceiling's technical design was performed, and a prototype was realized in this research. The experimental verification of the proposed system was performed to determine the actual parameters in different operational modes and their comparison with the same cooling ceiling system without PCM. The transient simulations in the TRNSYS simulation software were also carried out and validated with experiments. Based on the findings, the complex simulations were performed for application under the specific climatic conditions of Czechia. This work contains a comprehensive approach from the prototype design, through the experiments and partial simulations, to complex simulations based on the specific parameters and an economic evaluation. The results of this work show the ability of the proposed cooling ceiling solution to reduce temperature peaks by up to 3.2 °C, while energy savings can be up to 27%, depending on the air change rate. These results can also help significantly determine the economic feasibility of a PCM-based solution for ceiling cooling systems.
查看更多>>摘要：This study examines wetting behaviour and evaporation heat transfer of a falling liquid film inside a circular tube carrying grooves at its internal surface. The experiments include five different structures (four helical, one longitudinal) and one smooth tube (Cu, di≈27mm) with counter-current flow of liquid (downwards) and vapour (upwards). Results are obtained for liquid propane films (Pr=2.79, Resmooth tube=160?645) at different twisting angles and height of the grooves, tube inclination angles (β=0°?20° from the vertical), mass flow rates (from 5kg/h up to 20kg/h) and driving temperature differences (2K, 3K and 4K) mainly at a film inlet temperature of 293.15K. For wetting behaviour investigations the liquid film is visually observed at the lower end of the tube. The experiments reveal that the internal structures used lead to a strong improvement in both wetting behaviour and heat transfer, even up to inclination angles of 20°. The amount of capillary transported liquid within the grooves coupled with groove geometry itself are the main impacts on wetting behaviour and will be discussed within this paper.
查看更多>>摘要：With regard to the arrangement of air conditioning and ventilation in a building with large spaces, the uniformity of airflow distribution is often related to the air supply mode, heat flux, and location of the heat source. In this study, a 1:38 small-scale model of a large-scale sphere-shaped facility with high heat flux dissipation was constructed for investigating and analyzing the airflow distribution in the upper occupied zone. Additionally, a series of environmental parameters was experimentally obtained to characterize the influence of the location and release rate of the heat source. A dimensionless method was used to process the acquired data and explore the airflow characteristics. The experimental results revealed the evident influence of the release rate and heat source positions on the temperature distribution in the upper occupied zone. Furthermore, a reference temperature non-uniformity coefficient was proposed to assess the temperature uniformity in the occupied zone. Ultimately, the coefficients of temperature non-uniformity, reference temperature non-uniformity, and energy efficiency were reliably predicted. Consequently, a supply air temperature of 18 °C was selected as the optimal design parameter after conducting experiments to maintain the upper occupied zone temperature at 21 ± 1 °C and the corresponding minimum airflow rate could significantly reduce the energy consumption of air processing.
查看更多>>摘要：An external hydronic heating system utilizing geothermal energy is an innovative new method to de-ice in-service bridges that avoids detrimental environmental effects and damage to bridge deck structures stemming from chemical deicers and mechanical traction materials (e.g., sand). However, in these systems, the bottom surface of the bridge deck is usually fully covered by insulation materials and hence not accessible for visual bridge inspection. In this study, a new attached insulated, cross-linked polyethylene (PEX) pipe loop heating system is developed to heat existing bridges while maintaining a partly uncovered deck for visual inspection. The new loop consists of a PEX pipe, an aluminum plate, polyurethane foam insulation, and a high-density polyethylene jacket. Numerical simulations were first performed to determine the optimum shape and thickness of the insulation. Then the pre-made loop system was attached to the bottom surface of a concrete slab in a freezer box, where 62.3% of the slab bottom surface was covered by the system. A heated water bath was used to simulate the ground heat exchanger for heating the system. The heating tests were conducted at room temperatures of ?2.78, ?3.89, and ?6.67 °C, with supplied water temperatures ranging from 24 to 45 °C. A 1-D steady-state thermal resistance model was then created to model the heat transfer processes and to estimate the slab surface temperatures. The results show that the deck surface heat flux fluctuated from 49.6 to 121.5 W/m2, depending on operating conditions and that an average of 16.3% of heat energy was transferred to the deck top surface. The results of misting tests indicate that the new system kept the bridge deck free from ice under mild winter conditions. These results demonstrate that the new insulated PEX pipe loop heating system is feasible for deicing existing bridges while minimizing the hindrance of bridge inspections.
查看更多>>摘要：Accurate prediction of wellbore temperature plays an extremely important role in the production design of deep-water gas wells. This study overcomes the shortcomings of the previous literature in temperature calculations, comprehensively studies the heat transfer mechanism in the annulus and tubing, and an improved thermal model for predicting wellbore temperature in deep-water gas well is established. This model includes two important innovations: the natural convection heat transfer model in the annulus and the gas–liquid two-phase heat transfer model in the tubing. Firstly, the mechanism of heat transfer through natural convection in annulus is revealed by a numerical study, and a new calculation method for the annulus convective-heat-transfer coefficient, which can be applied to non-Newtonian fluids, is proposed. Secondly, considering that the two-phase flow in the tubing often presents an annular-mist flow in water-bearing gas well, a heat transfer model of the annular-mist flow in the tubing is developed considering the gas–liquid distribution characteristics and droplet dynamics. The results obtained using this improved thermal model are in good agreement with the field data of deep-water gas well, and the model prediction error is within 10%. Further, the sensitivity analysis is carried out on the parameters of annulus testing fluid that affect the wellbore temperature, and a novel solution for the annular insulation testing fluid was designed. Compared with ordinary annulus testing fluids, the wellbore temperature are more than doubled after adopting the new solution, thereby providing effective guidance for the development of insulation testing fluid system during deep-water gas well testing.
查看更多>>摘要：Data-driven model is considered to be an efficient and convenient diagnosis method for refrigeration systems, especially in Big Data Era, but the black box characteristic has always been a criticism and hindered its application. For this problem, SHapley Additive exPlanation (SHAP) method is explored to unveil the black box and the knowledge of system-level chiller faults has been mined. Two ensemble models with excellent diagnostic performance have been investigated including Random Forest (RF) and Light Gradient Boosting Machine (LightGBM). Detailed analysis shows that the sub-cooling at the condenser outlet (TRC_sub) and the condenser approaching temperature (TCA) have a great contribution to the detection of refrigerant leakage and overcharge with different margin, which is consistent with but standing out from the prior knowledge. In practical applications, temperature and pressure sensors for lubricating oil are recommended for a refrigeration system with lubricant due to the sensitive variation with system change (failure or condition altering), which can be a supplement to the traditional experience. The data-driven model is, indeed, an effective way to discover the hidden knowledge of a data set from all dimensions to upgrade the traditional experience. Furthermore, the local explanation by individual samples and the explanation comparison between the RF and the LightGBM model show that the primary features on which the model(s) depends to diagnose the same fault are basically the same, while the auxiliary features may vary slightly. The consensus further proves that the primary features and the related rules mined out in this study can be trusted.
查看更多>>摘要：In order to effectively utilize the pressure energy and cold energy of the natural gas pressure reduction stations, this paper proposes a comprehensive energy utilization system for natural gas pressure reduction stations based on Allam cycle. The low-grade waste heat during the Allam cycle and ambient heat are used to heat the decompressed natural gas to the temperature required by the user. Based on the established mathematical model, the effects of turbine inlet temperature, turbine inlet pressure, turbine outlet pressure, isentropic efficiency of power equipment, and natural gas expansion stages on the performance of the proposed system are studied. The proposed system and the pre-heating system are compared and analyzed. The results show that when the combustor outlet temperature is 900℃, the energy efficiency and incremental efficiency of the proposed system are 99.31% and 103.55%, respectively. Compared with pre-heating system based on gas turbine cycle, they have increased by 9.84 percentage points and 19.32 percentage points, respectively. Compared with the boiler-based post-heating system, the energy efficiency is increased by 3.5 percentage points.
查看更多>>摘要：Gas-liquid two-phase jet condensation has a property of high heat and mass transfer efficiency, thereby its theory and method becoming an urgent requirement for optimization design and safe operation of various industrial processes. Based on the experimental study of steam jet condensation in subcooled water flow in a vertical pipe, we have developed a numerical simulation method of steam-water two-phase jet condensation coupled with thermal phase change model and two-fluid model. Meanwhile, we emphasize on the shape of the steam plume, the distribution of pressure and velocity, and the heat and mass transfer of the two-phase interface. As the steam mass flux increases from subsonic to sonic, the steam plume becomes longer, and its shape transforms from cone to cylinder-cone gradually. The steam volume fraction is close to 0.01 at the end of the two-phase region, where the total pressure in the centerline approaches the total pressure of the water and reaches the maximum. The mass transfer process of steam condensed into liquid mainly occurs in the gas–liquid two-phase region, accompanied by drastic parameter change in pressure and velocity. Under the working conditions of this paper, the heat transfer coefficients based on the interfacial area of the steam plume and the volume of the two-phase region range from 1.71 MW/m2 K to 3.51 MW/m2 K, and from 2398.80 MW/m3 K to 5040.07 MW/m3 K, respectively.
查看更多>>摘要：Microchannel Heat Sink (MCHS) has been widely adopted in thermal engineering fields, such as refrigerators, chip cooling, battery packs, etc. To meet the ever-increasing demand for heat dissipation, surface modification methods adopting micro/nanoscale-modified surfaces have received considerable attention. In this paper, an advanced micro/nanoscale surface modification design is proposed based on a Lattice Boltzmann method (LBM) simulation study. Coupled boundary treatments at the inlet/outlet are developed with better numerical stability. The effects of surface wettability and micropillar on MCHS heat transfer performance are analyzed through bubbles' dynamic behaviors, Nusselt number, heat flux, and pressure drop. Design-based suggestions are proposed, and the enhancement mechanisms are explained. Results show that hydrophobic surface is more preferred for temperature-sensitive devices with low superheat requirement (Ja<0.1115), while the hydrophilic surface is more preferred for devices with a large heat dissipation requirement (Ja≥0.1286). Furthermore, the micropillar surfaces with pillar geometric factor Sp of 7 can yield the optimum heat transfer performance under a wide range of superheat conditions. Finally, an advanced design of a biphilic micropillar surface is proposed with superhydrophobic regions located at the top of the pillars, and other regions remain hydrophilic surfaces. An excellent heat transfer enhancement of 105.8% is achieved even compared with a pure hydrophilic micropillar surface. The enhancement is attributed to the superhydrophobic top regions efficiently blocking the bubbles merging process, which leads to more intense bubbles departure. These results provide a valuable guide for MCHS design and unravel the enhancement mechanism of the flow boiling process.
NSTL
Elsevier