Heat Exchange Area Research Articles

Thermal Operation Maps for Lamm–Honigmann Thermo-Chemical Energy Storage—A Quasi-Stationary Model for Process Analysis

The Lamm–Honigmann energy storage is a sorption-based storage that can be arbitrarily charged and discharged with both heat and electrical power. The mechanical charging and discharging processes of this storage are characterized by an internal heat transfer between the main components, absorber/desorber and evaporator/condenser, that is driven by the working-fluid mass transferred between those components with the help of an expansion or compression device, respectively. In this paper, thermal operation maps for the mechanical charging and discharging processes are developed from energy balances in order to predict power output and storage efficiency depending on the system state, which, in particular, is defined by the mass flow rate of vapor and the salt mass fraction of the absorbent. The conducted method is applied for the working-fluid pair LiBr/H2O. In a first step, a thermal efficiency is defined to account for second-order losses due to the internal heat transfer; e.g., for discharging from a salt mass fraction of 0.7 to one of 0.5 (kg LiBr)/(kg sol.) at a temperature of 130 °C, it is found that the reversible shaft work output is reduced by 1.1–2.9%/(K driving temperature difference). For lower operating temperatures, the reduction is larger; e.g., at 80 °C, the efficiency loss due to heat transfer rises to 3.5%/K for a salt mass fraction of 0.5 (kg LiBr)/(kg sol.). In a second step, a quasi-stationary assumption leads to the thermal operation map from which the discharging characteristics can be found; e.g., at an operating temperature of 130 °C for a constant power output of 0.4 kW/m2 heat exchanger area at volumetric and inner machine efficiencies of ηi=ηvol=0.8 and for an overall heat-transfer coefficient of 1500 W/(K m2), the mass flow rate has to rise continuously from 1.5 to 4.2 g/(s m2), while the thermal efficiency is reduced from 97% to 83% due to this rise and due to the dilution of the sorbent. For this discharging scenario, the corresponding discharge time is 4.4 (min·m2)/(kg salt). This results in an exergetic storage density of around 29 Wh/(kg salt mass). For a charge-to-discharge ratio of 2 (charging times equals two times discharging time) and with the same heat-transfer characteristic and machine efficiencies for constant power charging with adiabatic compression, the system is charged at around 0.75 kW/m2, resulting in a round-trip efficiency of around 27%. Besides those predictions for arbitrary charging and discharging scenarios, the derived thermal maps are especially useful for the dimensioning of the storage system and for the development of control strategies. It has to be noted that the operation maps do not illustrate the transient behavior of the system but its quasi-stationary state. However, it is shown, mathematically, that the system tends to return to this state when disturbed.

Leading edge impingement cooling analysis with separators of a real gas turbine blade

• Experiments of impingement cooling at real temperature conditions are conducted. • A new cooling design containing separators is suggested to prevent cross flow. • Deviation by using low temperature conditions of impingement cooling is presented. • Heat transfer data of impingement cooling are compared with published studies. This paper reports an analysis on leading edge impingement cooling of a turbine blade under real operation conditions of gas turbine. In the experiments, both mainstream and cooling air are heated to real high temperatures, and overall cooling effectiveness on the external blade mid-span surface is measured. Numerical strategy including RNG k-ε turbulence model is validated by the experimental data. Impingement cooling models with and without separators (Model A and Model B) are used as specimens, and the effects of high temperature conditions, separators and temperature ratios are discussed. The results indicate that: 1) When jet Reynolds number, mass flow ratio and temperature ratio are matched to the real conditions, the Nusselt numbers predicted by low temperature conditions can be generally matched, exclude the forepart of the internal surface at the blade leading edge. But the error of overall cooling effectiveness on the blade external surface is not neglected. 2) The application of separators can prevent the heat transfer deterioration caused by cross flow with extending the heat exchange area, thus the leading edge with separators can be further cooled over 30 K compared to the traditional design. 3) Under the real operating conditions of gas turbine, two new correlations between the surface averaged Nusselt number with jet Reynolds number and temperature ratio are suggested.

A research on silver powder sinters for dilution refrigerator heat exchangers

Due to the Kapitza resistivity, a sharp deterioration of heat transfer occurs between solid and liquid in the heat exchanger of the dilution refrigerator at extremely low temperature. It is necessary to use silver powder sintered heat exchangers to optimize the interface heat transfer. A theoretical calculation of heat exchangers at extremely low temperature was carried out to analyze the influence of the Kapitza resistivity on heat transfer performance. Silver powder with different particle sizes of 80 nm, 200 nm and 500 nm were selected for the preparation of sinters. Their micro-scale sintering conditions, pore volumes and specific surface area were carefully presented. It can be concluded from the theoretical calculations and the experimental results that the 200 nm silver powder sinters can optimize the performance of dilution refrigerator heat exchanger with a large heat exchange area.

A Review Paper on Computational Study of Earth Tube Heat Exchanger System

Abstract: Earth Tube heat exchanger can be proved as a better substitution to the space heating and HVAC problems that we face in our day to day lives. Earth tube heat exchangers can work in two phases, i.e. as a heater during winter seasons and as a cooler during summer season. ETHE can work more efficiently than traditional HVAC. This project concentrates on computational study of earth tube heat exchanger. The designing calculations were done for appropriate length and area of an earth tube heat exchanger. A model of ETHE was produced on basis of this calculations and dimensions obtained. The model was generated using Solidworks R2021 software. The ETHE model generated was then analysed for thermal stresses, distribution and other parameters. A CFD simulation of model for velocity and flow was carried on. The conclusions obtained show that ETHE works efficiently and can be used as a substitution for traditional HVAC. Keywords: Heating ventilation and air conditioning(HVAC), Earth tube heat exchanger(ETHE), Computational fluid Dynamics (CFD), Efficiently, Thermal stresses.

Numerical Simulation and Economic Evaluation of Wellbore Self-Circulation for Heat Extraction Using Cluster Horizontal Wells

The heat extraction capacity of the self-circulation wellbore is usually small because of the limited heat exchange area. In the paper, the cluster horizontal well group technology was proposed to enhance the heat extraction capacity and decrease the unit cost. Based on the mathematical model of heat transfer, a numerical simulation model of wellbore self-circulation for heat extraction using cluster horizontal wells was established to study the influence of main factors on heat extraction capacity. The economic analysis of heat extraction and power generation was carried out according to the model of the levelized cost of energy. The results show that the enhancement of heat extraction capacity is limited after the injection rate exceeds 432 m3/d (1.59 MW/well). The inflection point of the injection rate can be determined as the design basis for injection-production parameters. When the thermal conductivity of formation increases from 2 to 3.5 W/(m·K), the heat extraction rate will increase 1.45 times, indicating that the sandstone reservoirs with good thermal conductivity can be preferred as the heat extraction site. It is recommended that the well spacing of cluster wells is larger than 50 m to avoid the phenomenon of thermal short circuit between wells, and the thermal conductivity of the tubing should be less than 0.035 W/(m·K) to reduce the heat loss of heat-carrying fluid in the tubing. Compared with a single well, a cluster horizontal well group can reduce the unit cost of heat extraction and power generation by 24.3% and 25.5%, respectively. The economy can also be improved by optimizing heat-carrying fluids and retrofitting existing wells.

Mechanism of the heat exchange promotion by superimposing the external sound wave in a standing-wave thermoacoustic system

For improvement of energy conversion efficiency, a sound wave is superimposed with a loudspeaker to the working fluid in the stack. By using this method the work-flow generation of the stack was enhanced. To analyze this enhancement mechanism, the thickness of the boundary layer and the heat exchange area in the stack are calculated from the viewpoint of the heat exchange circumstance. The effect of the heat exchange circumstance on the particle displacement and thermal flow is investigated. As a result, it is confirmed that the superimposed sound wave improves the heat exchange circumstance and then the thermoacoustic phenomenon is enhanced.

Impact of Air Density Variation on a Simulated Earth-to-Air Heat Exchanger’s Performance

Due to their simple design and reliable operation, earth-to-air heat exchangers (EAHE) are used in modern buildings to reduce ventilation heat losses. EAHE operation in atmospheric conditions results in variation in ambient air temperature and pressure affecting air density. The paper presents the study on the impact of ambient air density variation on the calculated hourly air temperature at the EAHE outlet and the resulting energy use for space heating and cooling of an exemplary residential building. The ground temperature was computed from the model given in EN 16798-5-1. Then, air density was obtained using five various methods. Energy use for space heating and cooling of the building was computed using the 5R1C thermal network model of EN ISO 13790. Depending on the chosen method and concerning the base case without EAHE, a reduction in annual heating and cooling needs was obtained from 7.5% to 8.8% in heating and from 15.3% to 19% in cooling. Annual heating and cooling gain from EAHE were 600.9 kWh and 628.3 kWh for heating and 616.9 kWh and 603.5 kWh for cooling for the Typical Meteorological Years (TMY) and International Weather for Energy Calculation (IWEC) files, respectively. Unit heating and cooling gains per heat exchanger area were from 34.9 kWh/m2 to 36.8 kWh/m2 and from −35.1 kWh/m2 to −36.3 kWh/m2. Density variation with temperature from the relevant typical Polish meteorological year at constant pressure, in comparison to the method of EN 16798-5-1, resulted in an hourly difference of that unit gain up to 4.3 W/m2 and 2.0 W/m2 for heating and cooling, respectively. The same was true inthe case of IWEC files that resulted in differences of 5.5 W/m2 and 1.1 W/m2.

Novel C/SiC porous ceramic with controllable properties served as transpiration cooling material

Transpiration cooling has been gradually regarded as one of the most efficient and potential thermal protection technologies for the high temperature hot-end components of future hypersonic vehicle. The cooling performance of this technology profoundly depends on the properties of porous medium. This paper exhibits a novel C/SiC porous ceramic with controllable properties adjusted by molding pressure and carbon fiber content in the grinding-mold pressing-sintering process, and investigates the transpiration cooling performance using this ceramic with liquid water. Molding pressure improves from 50 MPa to 100 MPa, the compressive strength has increased by 52.9% and 67.9%, and the permeability decreases about 38% and 39.5% respectively corresponding to fiber content of 6% and 10%. The surface temperatures and back-side temperatures of porous ceramic with the largest molding pressure reaches the highest due to the lowest porosity which can reduce the heat exchange area between cooling water and porous ceramic skeleton. This novel porous ceramic with controllable characteristics can optimize the design of the transpiration cooling system.

Performance Evaluation Criteria for Triangular Microchannels with Smoothed Corners

The manufacturing capabilities available today for microchannels make it possible to produce in a comparatively easy, quick and cheap way several cross-sections, possibly allowing modifications of the macro-geometry. Another way is smoothing the corners of polygona cross-sections: this eliminates low-gradient areas and increases transport phenomena, i.e. frictional losses and heat transfer. Several ways of assessing the relative performance of the new shape in comparison to the original have been suggested over the years, among which are Performance Evaluation Criteria (PEC), as proposed by Bergles and Webb. PEC are based on a first-law analysis and aim at extremising the thermal power, heat exchange area, inlet temperature difference or pumping power under varying constraints. In this work equilateral triangular microchannels with uniform wall temperature are considered, through which a liquid flows in fully-developed, steady laminar regime. The cross-sectional area has its corners progressively rounded, and the velocity and temperature profiles are determined, in order to compute the Poiseuille, Nusselt and Stanton numbers, which are then employed in computing the objective functions for some PEC.

Experimental investigation on thermal management of lithium-ion battery with roll bond liquid cooling plate

• A liquid cooling plate with flexible flow channel design is fabricated using roll bond process, its thermal performance is verified experimentally. • Using proposed liquid cooling plate, the temperature of battery module is below 35 °C, and the temperature difference is within 5 °C at 2C discharge rate (750 W). • Compared with other studies, roll bond liquid cooling plate has strong heat transfer capacity, light weight and low cost. • Experimental results show that with Re increases, the improvement of the thermal performance of rib is greater than cavity. Lithium-ion batteries are widely used in energy storage systems owing to their high energy storage density, high energy storage efficiency, and stability. However, the power density of energy storage system is usually limited by thermal management. In this paper, the temperature distribution of the battery along the height direction is obtained. The thermal management analysis of two 100Ah lithium-ion batteries in series is carried out by using roll bond liquid cooling plate which has significant heat dissipation performance and low manufacturing cost. Cavity and rib structures are embedded in the serpentine channel and compared to the direct channel. Effects of flow rate and contact area are studied. The experimental results show that when charging and discharging at 1C, the cell temperature can be controlled by a low flow rate(12L/h) and the cavity structure with a large heat exchange area is dominant. In 2C discharge, large flow rate(>20L/h) is required. Rib structure with high flow rate and low water temperature has better performance. When discharged at 2C rate(750 W), roll bond liquid cooling plate can control the battery temperature below 35 °C and the temperature difference within 5 °C, at the cost of small pressure drop(5818 Pa). Compared with other studies, roll bond liquid cooling plate has strong heat transfer capacity, light weight and low cost, which is a promising solution for thermal management of energy storage batteries.

Sensitivity analysis and optimization of design parameters of heating tower heat pump

Heating tower heat pumps (HTHPs) are evolving rapidly owing to their high performance and adaptability. However, the lack of relevant design standards causes mismatches between components and leads to low operation efficiency. To address this problem, this study proposes a framework to optimize the design parameters of HTHPs. Case studies in 21 locations in different climate regions are carried out to investigate the saving in energy and costs. The results suggest that an increase in the design airflow flux (Ga), solution flow flux (Gs), water flow flux (Gw), or heat exchange area of the tower (A) can improve the performance of the heat pump unit and reduce the regeneration load. However, they can increase the energy consumption of the fan and pump, or the initial cost of the components. Considering 10-year lifecycle costs as the criterion, the optimal combination of Ga, Gs, Gw, and A is determined from 15,625 matrices with varying design parameters for each location. For the locations whose mean relative humidity (MRH) is approximately 70% during winter operations (e.g., Nanjing and Shanghai), the optimal design approach and range in winter are approximately 2.6 and 2.9 °C, respectively; these change to 4.7 and 6.6 °C in summer conditions. The Gs and Gw are 3.26 and 3.24 kg m−2 s−1, respectively, and Ga is 1.08 kg m−2 s−1 in summer and 2.94 kg m−2 s−1 in winter. The areas with MRH around 80% (e.g., Chongqing and Changsha) should have larger Ga and A, and lower Gs in winter, which can significantly reduce the energy used in regeneration. The regions with MRH less than 65% (e.g., Tianjin and Beijing) should have smaller Ga and A, and higher Gs in winter, which can reduce the negative effects of evaporative cooling. Compared with the conventional design parameter matrices, the optimal ones can save 7.01%, 9.91%, and 3.81% lifecycle costs, respectively, in the above three types of climate regions.

Concept and evaluation of energy-efficient boil-off gas reliquefiers in LNG carrier ships propelled by dual-fuel engines

Ships carrying liquefied natural gas (LNG) are generally propelled by dual-fuel enginesthat use boil-off gas (BOG) or diesel as fuel. Fuelling the engine with pumped and vaporized LNG is operationally more convenient than compressing BOG. BOG is usually reliquefied using reverse Brayton cycle (RBC) or Claude cycles. In this paper, both these cycles are reconfigured to extract the cold of pumped LNG to reduce power consumption. Steady-state simulations are done on process simulator Aspen HYSYS V8.6®. Capturing the cold from the LNG fuel improves the exergy efficiency of RBC and Claude reliquefiers from 37.8% to 57.5% and 48.5% to 75.6% respectively. The surface area of heat exchangers in the Claude system is one-third that of RBC. Mass flow through compressor and turbine in Claude system is one-fifth and one-ninth of RBC, while volume flow at turbine inlet is only one-fourteenth in the Claude system.The evaluation shows that all equipment needs to be designed with sufficientoperational flexibility to enable the system toextract cold effectively from the LNG fuel supply because it varies widely during the voyage. Claude system with cold extraction from LNG fuel is highly compact and the most efficient reliquefier designed so far.

Stochastic bi-objective optimization for closed wet cooling tower systems based on a simplified analytical model

A fast and accurate representation of the complex heat and mass transfer processes of closed-wet cooling tower (CWCT) systems for optimal design and management is challenging, especially considering the impact of weather variability. This study develops a systematic approach for the stochastic bi-objective optimization of CWCT systems based on a simplified analytical model. The analytical model is constructed by using the effectiveness-number of transfer unit method, which is embedded in a stochastic non-linear programming model for optimizing two competing objectives, namely the specific area and the coefficient of performance. A decomposition strategy is proposed to convert the original problem into multiple bi-objective optimization sub-problems by sequentially discretizing the design and uncertain spaces. Each bi-objective optimization sub-problem is solved using an augmented e-constraint method. The resulting solutions are finally processed by an efficient multi-criteria decision-making method, namely the Technique for Order Preference by Similarity to Ideal Solution to select the preferred Pareto solution. From an illustrative case study, it is found that the stochastic approach outperforms the deterministic approach since it allows for a certain design margin of heat exchanger area to reduce the impact of the variability in weather and to avoid the risks of abnormal running cases.

Methods for geothermal resource assessment of hot dry rock: A case study in the Gonghe Basin, China

Hot dry rock (HDR) geothermal resources are renewable energy source. Many of the findings of HDR resource evaluations have been used in energy planning and EGS design. However, to assess the amount of HDR resources in different locations, a consistent classification scheme and evaluation methods are still lacking. Considering geological credibility and economic feasibility, HDR resources are separated into three categories: vision, reserve, and exploitable. Vision and reserve are stationary resources that can be evaluated using the volume technique, and the exploitable resources can be evaluated using the numerical simulation approach. The HDR vision resource of the Gonghe Basin is evaluated to be 4.076 × 1022 J, and the reserve resource of the Qiabuqia HDR mass is evaluated to be 2.11 × 1020 J. At the Qiabuqia HDR development site, a discrete fracture network (DFN) model is applied for numerical simulation computations, which is based on the notion of local thermal nonequilibrium. The K1 and K2 wells produce varying amounts of heat due to the heterogeneous features of the fractured medium model, which is primarily due to differences in fracture density, heat exchange area, and fluid migration pattern. The categorization system and assessment technique can be used as a guide for evaluating HDR resources in the future.

Thermal and Economic Analysis of Heat Exchangers as Part of a Geothermal District Heating Scheme in the Cheshire Basin, UK

Heat exchangers are vital to any geothermal system looking to use direct heat supplied via a district heat network. Attention on geothermal schemes in the UK has been growing, with minimal attention on the performance of heat exchangers. In this study, different types of heat exchangers are analysed for the Cheshire Basin as a case study, specifically the Crewe area, to establish their effectiveness and optimal heat transfer area. The results indicate that counter-current flow heat exchangers have a higher effectiveness than co-current heat exchangers. Optimisation of the heat exchange area can produce total savings of £43.06 million and £71.5 million, over a 25-year lifetime, in comparison with a fossil-fuelled district heat network using geothermal fluid input temperatures of 67 °C and 86 °C, respectively.

Evaluation of the CBC-ORC energy system in lunar base: Working fluid combination selection, day and night operation performance

The lunar energy system must be able to provide continuous power supply on the day and night. However, the moon night is as long as 14 days, and the solar power generation system can’t work without solar radiation. The solution is that part of the heat in the daytime is used for storage, the other part is used for daytime power generation, and the heat stored in the daytime is used for power generation at night. Based on this, this paper proposes a closed Brayton cycle and organic Rankine cycle combined power generation system, and explores the operation performance of the composite power generation system in the whole lunar day process. The results show that the combination of He-Xe and toluene has the best performance among many working medium combinations. The thermal efficiency of the system can reach 33.82 %, and the heat exchange area of the radiator is 970.99 m2. Combined with supercritical CO2, the thermal efficiency is increased by 5.2 % and the heat exchange area is reduced by 8.5 %. In addition, in the case of night, the Brayton cycle system stops working at 7.75 earth days, which can maximize the power generation capacity of the coupling system, with a power generation capacity of 94.8 MJ.

Performance analysis and working fluid selection of organic Rankine steam compression air conditioning driven by ship waste heat

In order to effectively use the waste heat of ship flue gas and cooling water, in this paper, the organic Rankine-vapor compression refrigeration system is used for air conditioning for ships, and the thermodynamic model of the system is established. The system performance corresponding to the five commonly used working fluids R22, R141b, R236ea, R218 and R601 is analyzed and compared. The working fluid is optimized with the goal of the best system performance. Calculations proves that: regardless of the organic Rankine cycle and vapor compression refrigeration cycle, or for the entire system, R601 is the most suitable working fluid. The temperature of the heat source and the condensing temperature have an important effect on the performance of the system. The hot water temperature can be controlled and adjusted by changing the hot water flow rate, thereby optimizing system performance. At the same time, the determination of the condensing temperature should comprehensively consider the system performance, cooling water pump power, heat exchanger area and system investment to achieve the best overall system performance.

Heat Transfer in Pulsating Laminar Flow in a Pipe: Evaluation of the Reduction in the Heat Exchange Area of Oil Cooler

In this study, the heat transfer in a pipe was investigated by a numerical method with the pulsating flow of oil at Reynolds numbers 280, 350, 420. The diameter D of the pipe was 0.014 m. The pipe length was 100D. The flow pulsations were not harmonical. A numerical experiment was carried out using AnsysFluent. Based on the data obtained from the numerical experiment, an estimate of the reduction in the heat exchange area of the oil cooler was calculated. The reduction in the heat exchange area of the oil cooler ranged from 2.57% to 8.63%, depending on the pulsation mode.

Efficacy of the number of plates, fluid flow rate and volume fractions of aluminum oxide nanoparticles on thermal performance of gasket plate heat exchanger

The analytical model using the Second Law of Thermodynamics is applied to determine the thermal performance of a gasket plate heat exchanger. The gasket plate heat exchanger under analysis was initially projected for milk pasteurization, with water as a refrigerant, a mass flow rate equal to 0.42 kg/s, and a few plates equal to 20. The present work aims to determine the thermal performance of the heat exchanger, varying the number of plates, the flow rate of the refrigerant fluid, and using nanofluid with variations of volume fractions of aluminum oxide. Thermal efficiency, thermal effectiveness, generation entropy rate, heat transfer rate, and refrigerant and milk outlet temperatures were determined for comparison. In addition, the physical configuration of the plate heat exchanger, concerning the size and heat exchange area of the plates, was maintained.

Аnаlysis of efficiency of using heаt pump units for heаt recovery from ventilаtion emissions

The system analysis of the current state of quality of functioning of ventilation systems of industrial and public buildings in terms of volumes and modes of fuel and energy resources consumption has been carried out. The analysis of energy efficiency of using a steam-compression heat pump system for utilization of heat of ventilation emissions for heating the supply air has been carried out. Direct-flow ventilation systems with and without the heat pump unit are compared. A mathematical model and a calculation algorithm of the considered ventilation systems are offered. The model can calculate operation modes of one of the arrangement options both with fixed values of parameters affecting the result and with variables, which allows carrying out multivariate calculations and numerical research quickly and with minimum labour inputs. The results of comparative analysis are given on the example of a specific production room. Heat, moisture and air balance data obtained from regulatory documents and reference data, as well as the results of research by other authors are used as source data. Depending on the chosen arrangement, the contents of the calculations and the results obtained are different.In addition, the developed model allows, depending on the considered arrangement and the need, to obtain a large number of output parameters, such as surface areas of heat exchangers, heat transfer coefficients, all thermophysical properties of working media in all nodal points of the flowing process, etc. The principles of a comprehensive approach to assessing the efficiency of ventilation systems based on a system analysis, taking into account the parameters of heat and cold consumers, environmental parameters, the impact of aerodynamic characteristics of the air duct on energy costs and the tariff index in different climatic zones of Russia are substantiated.