Applied Thermal Engineering最新文献

To investigate which shape of solid obstacles is most optimal in terms of detonation-assisting capability within a closed channel, this study utilizes the Large Eddy Simulation (LES) method, based on the OpenFOAM platform, to conduct a detailed numerical study on the effects of different shaped obstacles (rectangular, rhombus, trapezoidal, circular, triangular) on flame acceleration and detonation initiation processes. The results show that the obstacles arranged in the middle of the combustion chamber can create more passages and local flames. Meanwhile, changes in the shape of obstacles can also produce different vortex structures and recirculation zones. These differences affect the flame surface area and the total combustion heat release rate, thereby affecting the flame acceleration effect. Judging from the results, triangular obstacles have the best flame acceleration effect, followed by rhombus, trapezoid, rectangle, and circular obstacles. In terms of detonation initiation, the obstacles with a slope can induce vortex to detach earlier, which produce a better vortex-flame interaction. Simultaneously, the slope can reflect the shock wave more frequently, creating more favourable conditions for the formation of Mach reflection and Mach stem. Based on this, it is found that trapezoidal and triangular obstacles have better detonation-assisting capability. Additionally, it is found that the stronger the leading shock wave and the shorter the distance between it and the flame front, the better the detonation initiation effect. In general, the types of detonation initiation in this study all belong to the shock detonation transition (SDT). However, the detonation initiation process can be further classified into two categories: (I) Detonation induced by shock wave reflection; (II) Detonation triggered by shock wave focusing.

Commercial buildings are responsible for approximately 20 % of the total energy consumption and greenhouse gas emissions in the United States. Over 85 % of these buildings lack building automation systems, and many are small (<50,000 square feet), underserved, and use rooftop units (RTUs) for heating, ventilation, and air-conditioning needs. Because these buildings lack proper energy management systems, several operational deficiencies lead to excess energy consumption. Studies have shown that managing the RTUs’ heating and cooling set points, schedules, setbacks, and optimal start can result in a 20 % to 25 % reduction in electricity consumption in small commercial buildings. These buildings typically use fixed schedules to start the RTUs 60 to 120 min before occupancy begins, which results in excess energy consumption. This paper presents research that demonstrates and evaluates the performance of four optimal start methods, which utilize data-based modeling as a key element in facilitating adaptive control in response to time-varying inputs while requiring minimal sensor inputs. The evaluation found energy savings in two commercial buildings equipped with RTUs by periodically alternating four different optimal start models during the cooling and heating season. The resulting energy savings are positive for all models and range from 2 to 5 kWh/day/unit. The units on the east side of the building showed higher savings, while interior units showed greater variability in savings due to the differences in capacities and room sizes. Savings were considerably greater during the heating season compared to the cooling season. The performance of all four models on Mondays was poor; models suggested a shorter optimal start time, which resulted in relatively larger errors. The future work will look at using a different model for the days after weekends and holidays.

A comprehensive numerical and experimental study is conducted to investigate the flow dynamics and heat transfer characteristics of flows past a novel wavy-axis circular cylinder placed symmetrically between two parallel walls. For comparison, the flow past a corresponding straight-axis cylinder within the same bounded domain is also studied. The investigations were primarily conducted at Reynolds numbers of 626 and 680, based on the average velocity and diameter of the cylinder, with a blockage ratio of 0.42, based on the widest area of the channel. At these flow conditions, the straight cylinder expectedly showed a “reverse” von Kármán type wake. However, the special zigzag cylinder showed fundamentally distinct flow patterns at the wake leading to a significant drag reduction and a heat transfer enhancement, compared to the performance of the straight cylinder. The pressure drop of the zigzag cylinder is 14 % and 19.4 % lower than that of the straight cylinder, with a higher convective heat transfer coefficient of 24 % and 9 %, respectively, for Reynolds numbers of 626 and 680. The substantial drag reduction is attributed to the formation of steady flow pattern in the wake indicating the suppression of unsteady vortex shedding. The heat transfer enhancement was mainly caused by the formation of elongated vortical structures principally aligned in the streamwise direction. The creation of these streamwise vortex tubes tends to more efficiently mix the fluid between the near-wall area and the core region of the channel than the near-planar vortex shedding associated to the straight cylinder. It was shown that the 3D shape of the zigzag cylinder generates a spanwise velocity component, which results in the creation of a dominant streamwise vorticity component that eventually develops into sustained vortex tubes.

The high-temperature heat pump (HTHP) operates under severe conditions and there is a need for further research into its operating characteristics and performance optimization. In this study, the effect of three optimization strategies, namely vapor injection, subcooling and cascade type, on the system characteristics of the HTHP at varying ambient temperatures (T_am) was investigated. Subsequently, a comparative analysis focusing on energy, economy and environment was conducted. As the result shows: The introduction of vapor injection creates the presence of an optimal intermediate pressure and greatly improves the COP of HTHP by a range of 14.7–52.6 %, while the application of an economizer further elevates the COP by 0.6–7.0 %; The cascade cycle exists an optimal intermediate temperature and has operating performance advantages at lower T_am. As the condensing temperature increases from 120 °C to 130 °C to 140 °C, the turning T_am rises from 54 °C to 63 °C to 71 °C, respectively; HTHP requires a certain level of T_am to maintain a higher level of system performance to meet economic and environmental requirements. In the research framework, HTHP begins to show economic advantages at a T_am of 10 °C, while it can only generate benefits of emission reduction at a T_am of 65 °C or higher.

A compact model is developed to predict the thermal and hydrodynamic performance of a flat heat pipe with a rectangular grooved wick. The present model relies on the analytical solution to the energy equation in the wall and an equivalent heat transfer coefficient predicted using a computationally efficient iterative method. This efficient iterative method can also provide a framework for modeling other grooved or porous wick heat pipes for which analytical or semi-analytical solutions to the wall conduction, fluid flow, and film equations exist. Compared to prior numerical tools, the present modeling approach is computationally efficient, making it compelling for use in parametric and optimization studies. Instead of numerically solving a set of coupled differential equations, the present model considers only analytical and semi-analytical solutions for evaporation and condensation heat transfer rates. The non-discretized nature of the present model allows computations on a typical workstation to be completed within seconds as opposed to the hours required for prevalent numerical tools. The present model accounts for varying liquid fill volumes, geometry, and interfacial properties, such as surface tension and contact angle. The present model closely agrees with published numerical and experimental results for wall temperatures and maximum heat transfer rates. Parametric studies, which vary wall thermal conductivity, water contact angle, and groove dimensions are conducted on a previously experimentally investigated heat pipe to demonstrate the present model’s capabilities. The present model found that the maximum heat transfer rate of the heat pipe can be enhanced by about 15 and 20% by varying its wetting angle and groove dimensions, respectively.

To improve the output characteristics of seawater desalination-solar chimney power plants, enrich the theoretical research basis and promote its commercial application, this paper introduces the concept of multilayer flow channel, establishes four new types of multilayer collector flow channel, and constructs the corresponding heat and mass transfer mathematical models, revealing the mechanism of the influence of different flow channel and structural parameters on the system performance. The results showed that the surround-flow system extended the heat absorption time of the airflow, had two thermal storage layers, and had the best performance. The daily average power generation was 43.3 kW, and the average water production rate was 87.2 g/(h·m²), which was 132.8 % and 91.2 % higher than the traditional type’s, respectively, but had the longest start-up time. For the surround-flow system, the main reason for the flow loss increasing is swirling regions below the inlet and the transition section caused by the structure itself. Both the inlet width increasing and the roof’s inclination angle decreasing can increase power generation, with little impact on water production. Corner width increasing results in a significant decrease in water production, with little impact on power generation. The determination of structural parameters should also consider operational costs.

Heat transfer enhancement is particularly difficult to achieve within narrow space and turning channels due to strong structural constraints plus flow recirculations. Traditional designs usually follow periodic topology layout of simple geometric features, such as pin–fin arrays, to distribute cooling for narrow turning channels, lacking alignment with specific design objectives and the constraints. This is attributed to three main issues: the absence of control methods for complex structures, the lack of data for variable topologies, and insufficient understandings of Topologic-thermal Synergism. To address these challenges, the integration of self-organization geometry and the constrained Bayesian optimization method is employed. The channel is divided into 15 regions: the left region is subjected to solid presence control and anisotropy determination, while other regions are assigned control parameters, enabling self-organizing parameterization in design region. The parameters then are optimized using a constrained Bayesian optimization approach, aimed at maximizing the thermal performance factor (TPF) while constraining the area of low heat transfer regions. The optimization process begins with 216 samples to build the initial surrogate model, followed by 30 optimization iterations, resulting in the creation of high-performance complex structures and the establishment of a topology database. Furthermore, the SHAP method is utilized to extract topology design guidelines based on the most influential parameters and their beneficial variation directions. The guideline serves to guide the topology layout of other cooling structures in similar design scenarios. Results demonstrate that optimized self-organized structure enhanced the overall thermal parameter by 70% and decreased the low heat transfer zones by 74% when compared with pin fin structures. Data mining identified material orientation and generation on the left-side region as critical factors in topological structures, significantly influencing overall heat transfer performance. Leveraging the obtained design guidelines, a topological layout for a guiding pin fin structure is redesigned in wedge-shaped channels with varying contraction ratios, achieving performance improvements without the need for additional computational resources. The outcomes of this work hold scientific significance and industrial application value in enabling rapid design of high performance heat transfer structures.

This study presents numerical simulations and their validation for flow boiling of liquid nitrogen (LN₂) in a vertical upflow orientation, with a primary aim to understand the complex two-phase flow and heat transfer phenomena important to space applications. The computational fluid dynamics (CFD) model utilized the coupled level set volume of fluid (CLSVOF) method, incorporating additional source terms for bubble collision dispersion force and shear lift force in the momentum conservation equation to enhance simulation accuracy. The simulations were conducted for two mass velocities (G = 526 and 804 kg/m²s) and three different heat flux levels (approximately 10%, 30%, and 70% of critical heat flux (CHF) under Earth gravity. The model was validated against measured wall temperature data acquired from the authors’ previous experimental studies, demonstrating average deviations of less than 2.8 K across all operating conditions. The simulated two-phase flow contours illustrated various flow patterns, including bubbly, slug, churn, and annular. Both mass velocity and heat flux were observed to impact the onset of nucleate boiling (ONB), bubble nucleation, growth, and coalescence, and overall vapor structure. The simulations also offered insight into axial and radial void fraction and velocity profiles, revealing local flow acceleration trends synchronized with void fraction development. A comparison between predicted and measured bulk fluid temperature profiles showed excellent agreement, further validating the CFD model’s accuracy and practical usefulness for two-phase cryogenic flow boiling simulations in space applications.

This article presents a novel multi-channel condenser/radiator designed for mechanically pumped two-phase cooling systems in space. A numerical model coupling two-phase condensation heat transfer with thermal radiation is proposed to design and accurately predict the performance of the radiator, taking into account both single-phase and two-phase heat transfer occurring within the fluid channels. The performance assessment of the radiator is carried out in a thermal vacuum chamber with CO${}_2$ circulating through a closed cooling loop. The validation of the numerical model is confirmed by comparing the predicted results with the experimental data obtained. The comparison demonstrates good agreement with the discrepancy of 10% between the predictions and the actual measurements. The numerical method presented here is simpler compared to Computational Fluid Dynamics (CFD), making calculations more accessible, particularly for those without access to CFD tools. Additionally, this paper explores ways to simplify complex thermal radiation issues using surface-to-surface radiation models and the Monte Carlo method to calculate the view factor.

The valorization of waste heat to respond to the increase demand on electricity and cooling is an important energetic challenge. For this purpose, a hybrid thermochemical cycle is proposed. This discontinuous sorption cycle is based on reversible endothermic/exothermic solid–gas reactions, and it is able to recover medium grade waste heat (between 150 and 250 °C) to valorize it in a second step by providing cold production at its endothermic component and mechanical work thanks to the integration of an expander on the gas line. Moreover, the two-step operation leads to a storage functionality. This paper presents an experimental study of this new hybrid cycle prototype in different operating conditions, and a sensitivity study of the design parameters of the prototype performed through a steady state model. The experimentation shows the influence of the electrical load applied to the generator on the coupling between the expander and reactor, by affecting the rotational speed, the torque of the expander and the pressure of the reactor. The analysis of the experimental results highlighted the effect of two main limitations in the prototype: the internal leaks on the expander side (unlubricated scroll expander) and the heat transfer on the reactor side. A numerical parametric study is done on these two parameters, showing the possibility of cogenerating 125 W of mechanical power and 2.3 kW of cold for defined enhanced parameters of the prototype. These promising results prompt the next step of conducting a detailed exergetic analysis of the cycle. This analysis will aim to identify the primary sources of exergy destruction and to explore ways for reducing them by minimizing main irreversibilities in the cycle.