Heat Exchanger Effectiveness Research Articles

Determination of the sensible heat effectiveness and pressure loss of a rotary regenerative heat exchanger using CFD

The paper presents a new method to determine the sensible effectiveness and pressure loss of rotary regenerative heat exchangers using computational fluid dynamics (CFD). It is based on CFD simulations of a single thermal wheel microchannel with a small cross-sectional area and thin walls, with cyclic inlet and outlet boundary conditions. Two unique measurement set-ups were designed and built for the experimental measurement of the heat exchanger characteristics. Five different types of thermal wheels were manufactured, measured, and simulated in ANSYS Fluent. All wheels achieve an effectiveness greater than 73% under certain (air flow) conditions, which is the minimum effectiveness required by Ecodesign (in the EU). For the examined exchangers, the effectiveness ranges from 66.5% to 83.3%, depending on the boundary conditions and geometric parameters of the rotors. The highest sensible effectiveness is achieved by heat exchangers with the largest heat exchange surface Ac; on the other hand, these wheels have the largest pressure loss. The paper discusses the use of a simplified ε-NTU correlation model for the HVAC systems (typically C* = 1). The correlation model and CFD results were compared and found to be different from each other. The results of the CFD simulation were compared with measurements to prove that the proposed simulation method can predict the behaviour of the real heat exchanger as a whole. It was demonstrated that the sensible effectiveness and pressure loss of the rotary heat exchanger predicted by the CFD simulations correspond well to the measured values within the measurement uncertainty ±1.3%. The proposed method can be used for the comparison of different rotary regenerative heat exchangers before their manufacturing and for verification that they meet the EU Ecodesign requirements set by the current legislation. It reduces the cost of the initial optimisation and testing of new designs.

Enhancement of Double-Pipe Heat Exchanger Effectiveness by Using Porous Media and TiO2 Water

In this paper, the rate of heat transfer by forced convection in a counterflow heat exchanger, at turbulent flow conditions were investigated experimentally, using porous media and TiO2 Nanofluid to observe the behaviour of heat transfer with flow rate and volume concentration of nanoparticles t enhance heat transfer through it. 3 mm Steel balls (ε=39.12%) as a porous media completely filled to the inner pipe (core pipe). The cold and hot water are used as working fluids through the inner and outer pipes. Then using, the TiO2 nanofluid instead of cold water flowing through the porous pipe to enhance heat characteristics. The effects of operating parameters include flow rate (4 LPM, 6 LPM, and 8 LPM), Reynolds number between (3000 – 7000), and nanoparticle volume fraction (0.001, 0.002 and 0.003) on Convective heat transfer co-efficient and Nusselt number. Effective thermal conductivity is increased when the nanoparticle volume fraction is increased. The heat transfer coefficient increases with decreasing nanofluid temperature, but the heating fluid's temperature has no significant effect on the nanofluid's heat transfer coefficient. The results show that porous media and TiO2-based nanofluid's improve heat transfer at flow rate of 4 LPM by 35.4% and improve NTU and effectiveness at flow rate of 4LPM by 12.4%, and 24%, respectively, when compared to pure water without porous media. This improvement in thermophysical properties yielded high heat transfer of heat exchangers used in process industries.

Experimental thermal-hydraulic characteristics of single-phase natural circulation loop using water-based hybrid nanofluids

Nanoparticle shape strongly affects the thermal-hydraulic behavior of nanofluid-enhanced single-phase natural circulation loop (SPNCL) but is yet to be explored experimentally. Hence, various mono/hybrid nanofluids having dissimilar shaped nanoparticles (Al2O3+Water, Al2O3+CuO + Water, Al2O3+SiC + Water, Al2O3+MWCNT + Water) with a 0.1% volume concentration, are used in SPNCL to investigate mass flow rate (MFR), heat exchanger effectiveness (HEE) and total entropy generation rate (TEGR). The effect of power input, coolant inlet temperature and loop inclination (both counter-clockwise and clockwise) on transient and steady-state performances are presented. The result reveals that mono/hybrid nanofluid shows enhanced HEE and lower TEGR as compared to base fluids, whereas the MFR may increase or decrease since the performance of SPNCL is significantly impacted by the nanoparticle's shape. Spherical-shaped nanoparticle-dispersed nanofluids show early flow initiation and higher MFR than cylindrical-shaped nanoparticle-dispersed nanofluids. MFR and TEGR rise with higher power input, whereas HEE declines for all working fluids. MFR and TEGR increase as the coolant inlet temperature rises, whereas HEE decreases for all working fluids. The loop inclination decreases the MFR, while it increases the HEE and TEGR. The Counter-clockwise inclination of the loop shows a higher MFR than the clockwise inclination at the same angle of inclination.

Numerical study on dynamic performance of low temperature recuperator in a S-CO2 Brayton cycle

Transient response of the pinch point is crucial to the evaluations of the recuperator efficiency. A reheat and recompression cycle dynamic simulation system for supercritical CO2 is built, and a transient segmented model is developed to explore the dynamic characteristics of the recuperators. The operating conditions including inlet mass flowrate, temperature and pressure of the low temperature recuperator (LTR) change dramatically with the withdraw-mass variation, which causes different response characteristics of the pinch point. The pinch point gradually transfers from the cold end to the hot end of the LTR as the mass flowrate decreases. The higher pinch-point temperature difference leads to lower heat exchanger effectiveness, and the internal pinch point greatly weakens the recuperator performance. A new prediction model is proposed to quickly determine the pinch point location and analyze the operating conditions prone to cause internal pinch point. The range of split ratio in which the internal pinch point exists in the LTR is obtained. The lower-limit of split ratio variation is sensitive to the high- and low-pressures, the isentropic efficiency of the compressor, and the outlet temperature of the precooler, while the upper-limit of split ratio is only related to the pressures of the recuperator.

Numerical study on the flow maldistribution characteristics of a plate heat exchanger

Flow maldistribution problem in the plate heat exchanger precludes the intended ideal performance. In this study, the flow and temperature distribution impact on flow maldistribution depending on the channel number of the plate heat exchanger were investigated by computational fluid dynamics (CFD) analysis to identify the maldistribution flow effects of the plate heat exchanger and suitable installation location of the measurement device. Channels located at the end of the header are confirmed to have excessive flow, while the flow rate of working fluid in channels near the inlet pipe is relatively low. The channel flow distribution degraded with an increase in Ntot,ch. In plate heat exchangers withNtot,ch = 20, 40, and 60, γf,ch was 18–21.3 %, 24.3–25.9 %, and 29.1–31.2 %, respectively, whenG¯ch = 100–500 kg/(m2·s). A jet flow occurs when the fluid flows from the channel to the outlet header; therefore, the velocity and temperature distribution inside the outlet header is not uniform. However, the mass flux and temperature distribution in the pipe become uniform owing to the swirl flow. Thus, γvm=93.5 %–95.3 % and MATD = 0.26–0.33 °C whenG¯ch = 100–500 kg/(m2·s) at DPL = 10, which indicates that the velocity and temperature distribution of fluids become uniform over that point.

Heat exchange mechanism analysis and structural parameter optimization for series-combined microchannel heat sinks

This study presents a heat exchange mechanism analysis and an optimization of structural parameters of a built-in series combined microchannel heat dissipation platform, aiming to address the problems of low heat exchange and high pumping power caused by the mismatch in the selection of the structural parameters of the microchannel heat sink (MHS). A numerical calculation model was built in Fluent, and the temperature difference, pressure difference, and their ratio between the inlet and outlet of the MHS were used as the evaluation indicators. Using an orthogonal test, the influence of multiple parameters on the heat exchange effect was investigated. It is found that the shape of the microchannel is the main factor affecting the heat exchange phenomena, and the optimal combination of parameters such as the height, width, and length of the rectangular microchannel is obtained by a genetic algorithm. Physical transformation and heat exchange experiments were carried out. It was highlighted that the comprehensive performance of the optimized cooling system has been improved to varying degrees under different inlet flow conditions. At an inlet flow rate of 2.0 m3 h−1, the pressure difference and pumping power consumption are reduced by 24.9%, and the maximum temperature difference and heat dissipation power are increased by 21.0%. An experimental study on the heat exchange effect of MHSs with different numbers of serial pieces was carried out. With a number of MHSs of 15, the heat exchange is found to be sufficient, and the maximum heat exchange estimated was 9289.52 W.

CFD Simulation Study on the Air Side of a CO2 Evaporator in a Motor Train Unit Air Conditioning System

At present, China’s high-speed rail is in a period of rapid development. Most of the refrigerants used in Chinese motor train units at this stage are still R134a and R407c, which have an impact on the environment. In response to the environmental protection concept of green travel, it is of great significance to study the air conditioning system of motor train units using CO2 refrigerant. Using CFD simulation analysis technology, the heat transfer performance of the air side of the CO2 finned tube evaporator used in the air conditioning system of the motor train unit is studied. We select the air outlet temperature, pressure drop, heat transfer factor, and resistance factor as the objective function, in addition, monitoring points are set up in the air flow channel to monitor the turbulent flow field and pipe wall pressure. Our research shows that the cooling capacity of the CO2 evaporator can reach up to 29.76 kW, which can meet the heat exchange required in the air conditioning system of the motor train unit. In order to obtain a better structure and the conditions of the heat transfer effect, structural optimization was conducted. The simulation results demonstrate several trends: (i) With the augment of the air inlet velocity, the cooling capacity of the evaporator increases and the heat exchange effect improves; when the air inlet ve > 2.2 m/s, the effect of continuing to augment ve on heat exchange is weak. (ii) Following appropriate reduction of the diameter of the heat exchange tube, the wind resistance is reduced and the cooling capacity of the evaporator is improved. (iii) With the enlargement of the fin spacing, the turbulent motion in the flow channel can be fully developed, there is a peak in the change in the heat exchange tube area optimization factor, and the optimal fin spacing is between 1.6 mm and 1.7 mm; at this time, the average turbulent kinetic energy of the air side is larger and the turbulent dissipation rate is smaller. These results provide a reference for the practical application of CO2 refrigerant in the motor train unit.

Multi-objective thermo-economic optimisation of Joule-Brayton pumped thermal electricity storage systems: Role of working fluids and sensible heat storage materials

Pumped-thermal electricity storage (PTES), with the advantages of reduced geographical constraints, low capital costs, long lifetimes and flexible power ratings, is a promising large-scale energy storage technology for future power systems. In this work, thermo-economic models of Joule-Brayton PTES systems with solid thermal reservoirs (STRs) and liquid thermal stores (LTSs) were developed, and detailed parametric analyses of the two systems were performed. The results reveal that elevated maximum charging temperatures are beneficial for both thermodynamic and economic performance, and that there are optimal values for the packed-bed void fraction, heat-exchanger effectiveness and turbomachine polytropic pack from a thermo-economic perspective for the two PTES system variants. Multi-objective thermo-economic optimisation of PTES systems at a fixed power capacity (10 MW) and discharging duration (6 h) was also conducted. It is found that helium is the best working fluid candidate for both PTES systems, and that the best options for the storage material are magnetite for PTES systems with STRs, and the combination of Hitec XL + Therminol 66 + Butane for PTES systems with LTSs. In the investigated design space for both systems, PTES systems with STRs are more attractive as the total purchase cost is lower for the same roundtrip efficiency as PTES systems with LTSs. Using the technique for order of preference by similarity to the ideal solution decision-making method, and a selected weighted matrix (1:1), the optimal solutions amongst the Pareto front solutions were determined. The optimal roundtrip efficiency and total purchase cost are 71.8 % and 37.7 M$ for PTES systems with STRs, and are 56.0 % and 36.0 M$ for PTES systems with LTSs, respectively. The conclusions and proposed approach can provide useful guidance for the further development, design and optimisation of PTES technology.

COOLING PERFORMANCE PREDICTION OF A METAL FOAM INTERNAL HEAT EXCHANGER: AN ARTIFICIAL NEURAL NETWORK APPROACH

Although HFC refrigerants have high global warming potential (GWP) values, they are preferred due to their satisfactory cooling performance and A1 fire protection classification. If possible, alternatives of HFC-type refrigerants should be used; if not, they should be used with the least charge value. In this study, the effect of metal foam heat exchanger was investigated to reduce the amount of refrigerant in the refrigeration system. The performance of the metal foam incorporated internal heat exchanger (IHX) was estimated by trained artificial neural networks (ANNs) using the correlations given in the literature, and the results were compared with the experimental data presented in the literature. For the same cooling capacity, a higher performance is achieved by using IHX with metal foam additives. Although the developed correlation has been extracted for IHX, it could be applied for all HE with gas flow.

Improving the performance of the vapor compression cycle using an internal heat exchanger

The main additional consumer of electrical energy in an electric bus is the climate system, which can consume up to 30% of electrical energy. This problem is especially relevant in Russia at low ambient temperatures. The optimal solution is to switch the vapor compression unit to heat pump mode. However, at low temperatures, its efficiency decreases and performance is insufficient. One way to improve efficiency is to use an internal heat exchanger. In this paper, we studied the effect of an internal heat exchanger on the performance of a vapor compression cycle using R290, R410a and R507a refrigerants. The influence of geometry on the performance of the internal heat exchanger is determined. The use of an internal heat exchanger makes it possible to increase coefficient of performance by 10-13%.

Effect of an Internal Heat Exchanger on the Performances of a Double Evaporator Ejector Refrigeration Cycle

A theoretical investigation is presented about a double evaporator ejector refrigeration cycle (DEERC). Special attention is paid to take into account the influence of the sub-cooling and superheating effects induced by an internal heat exchanger (IHX). The ejector is introduced into the baseline cycle in order to mitigate the throttling process losses and increase the compressor suction pressure. Moreover, the IHX has the structure of a concentric counter-flow type heat exchanger and is intentionally used to ensure that the fluid at the compressor inlet is vapor. To assess accurately the influence of the IHX on the DEERC performance, a mathematical model is derived in the frame of the dominant one-dimensional theory for ejectors. The model also accounts for the friction effect in the ejector mixing section. The equations of this model are solved using an Engineering Equation Solver (EES) for different fluids. These are: R134a as baseline fluid and other environment friendly refrigerants used for comparison, namely, R1234yf, R1234ze, R600, R600a, R290, R717 and R1270. The simulation results show that the DEERC with an IHX can achieve COP (the coefficient of performance) improvements from 5.2 until 10%.

Exploratory computational fluid and particle dynamics analyses in multi-layered vertical duct-type ventilation system for heat exchange and air purification

This study aims to develop a vertical air duct which is embedded inside the building envelop. It is also designed as a counter-flow heat recovery ventilator (HRV). The vertical air duct model in this study adopted baffle structure considering two methods to suppress the inflow of the particulate matters from outdoor: 1) trapping by wall surface and airflow structure and 2) gravitational sedimentation in the air duct. Design parametric analyses are conducted using computational fluid dynamics to determine optimal design of vertical air duct. Three types of analytical case with various set-up of baffles inside air passages are prepared, and heat exchange efficiency is estimated by heat transfer analysis inside ventilator. In addition, particulate pollutants’ behaviour inside air passages is predicted based on Euler-Lagrange particle transport analysis. Through the series of numerical analysis, differences in heat exchange efficiency and effectiveness of particulate pollutant removal corresponding to different baffle design are quantitatively discussed, and finally optimal design for air passages in HRV is determined. The building-integrated HRV introduced in this study could contribute to sustainable design of building environment.

Internal heat exchange effect on the thermal decomposition of carbonates in the phosphorus-containing raw materials

Internal heat exchange effect on the thermal decomposition of carbonates in the phosphorus-containing raw materials

A model for computing thermally-driven shallow flows

In many natural disasters such as overland oil spills or lava flows, physical fluid properties as density change when considering non-homogeneous spatial and time variable distributions of the temperature. This effect is even more remarkable when these flows show a non-Newtonian behaviour due to the sensitivity of their rheological properties as viscosity or yield stress to temperature. In these cases, temperature becomes a significant variable that drives the fluid behaviour, which must be solved using an energy equation coupled with the free surface flow system. Special attention is devoted to thermal source terms which must include all the heat fluid exchanges, and their modelling sometimes can govern the complete flow behaviour. Fluid density, viscosity and yield stress, also affected by temperature, must be recomputed every time step. Summarizing, this work presents a 2D free surface flow model considering density and temperature variations, which could even modify viscosity and yield stress, with heat transfer mechanisms. The model is applied to oil spill overland simulations and heating/cooling test cases are carried out to ensure the system energy balance. As conclusions, it can be said that the numerical results demonstrate the importance of the heat exchange effects and those of the density, viscosity and yield stress variations.

Effects of Internal Heat Exchanger on Two-Stage Compression Trans-Critical CO2 Refrigeration Cycle Combined with Expander and Intercooling

Because of the limitations of traditional refrigerants, the application of trans-critical CO2 technology in domestic gas conditioners and other fields is becoming increasingly popular. This paper proposes a new CO2 trans-critical refrigeration system. Combining the internal heat exchanger and expander components, as well as the two-stage compression cycle, we analyzed the effectiveness of the expander, internal heat exchanger, and intercooling on system performance under various operating conditions in terms of energy, exergy analysis, and optimal discharge pressure. The system performance can be changed by changing the cycle conditions and internal heat exchanger effectiveness, which reduces system power consumption and the percentage of exergy losses of gas cooler components. Compared to the single-stage compression with expander cycle, the systems cycle power consumption is reduced by 2–15.7% and the maximum system COP is increased by 2.93–6.93%. From the view of energy effectiveness, the system’s maximum COP increases by 3.9% and the percentage of exergy losses of gas cooler decreases by 22.5% with the effectiveness of internal heat exchanger varying. The addition of an internal heat exchanger has resulted in improved system performance, which is important for providing a relevant cycle model for the application.

Design optimization of cross-counter flow compact heat exchanger for energy recovery ventilator

Energy recovery ventilators (ERVs) are the key equipment to fresh air ventilation, which is helpful for the control of respiratory diseases like COVID-19. In this paper, design optimization of the compact heat exchanger in a proposed heat recovery ventilator of the energy efficient building has been carried out and discussed. Appropriate theoretical models are required to evaluate system performance and potential energy savings. This is challenging because of the complexity of the preferred module combining cross- and counter-flow regions. The objective of the design optimization is to maximize the heat transfer effectiveness and to minimize the pressure loss of the compact heat exchanger with limited space. In this study, the allowable dimensions, heat transfer specifications and design requirements of the proposed heat exchanger are firstly defined. Then, the flow configuration, numbers, and dimensions of the air flow channels inside the heat exchanger are identified as the design parameters. A systematic design and optimization method for heat exchanger effectiveness improvement is explored. Furthermore, a detailed mathematical modeling is conducted and validated against the experimental results using the effectiveness-NTU method. It is found that the proposed modeling method is expected to be used to design of the compact heat exchanger. Finally, guidelines for improving the heat transfer effectiveness of air-to-air heat recovery ventilator were derived.

Flow and heat transfer performance of array finned channel coupled jet heat sink

The rapid rise in temperature of perovskite tandem solar cells under a high concentration ratio has introduced a huge decline in the efficiency of photoelectric conversion. To solve this problem, an array finned channel coupled jet heat sink has been designed. The ratio of height to diameter of the jet (H/d) on heat transfer, channel aspect ratio, pumping power, and other factors on heat transfer and flow characteristics are examined by the computational fluid dynamics and experimental methods. The results suggest that the trough micro-jet heat sink demonstrates better performance than the smooth cooled surface heat sink. The smaller the H/d, the better the heat transfer performance, and the larger the jet Reynolds number, the better the heat exchange effect. The key influence factor of pressure loss is the heat sink structure. With the increasing pumping power, the overall thermal resistance decreases, and the trend tends to be flatter when the pumping power exceeds 0.15 W. For constant Rea, the larger the input heat flux Q, the larger the energy efficiency ratio and the higher the utilization rate of energy.

Cooling Performance Prediction for Hydraulic Thermoelectric Radiant Cooling Panels with Experimental Validation

Thermoelectric technology has been developed as a substitute for existing refrigerants in heating, ventilation, and air-conditioning system applications for building decarbonization. A hydraulic thermoelectric radiant cooling panel (hTERCP) operated based on the Peltier effect can alternate a conventional cooling system using a chiller with refrigerators. This study aimed to develop a cooling performance prediction model for a hTERCP-integrated free cooling system according to the desirable range of five design factors. A mockup model of the hTERCP was constructed and tested in an environmental chamber to verify the proposed simulation model. The simulation and the experimental analysis confirmed that the heat rejection performance of the thermoelectric module (TEM) significantly affects the cooling performance of the hTERCP. The cooling water temperature was the primary design factor for releasing heat from the hot side of the TEM and significantly influenced the cooling performance of the hTERCP. A parametric analysis of the five design factors was conducted to investigate a method for improving the coefficient of performance (COP) of the hTERCP. The cooling water temperature affected the COP by 38.6–45.7%, and the heat exchange area of the cooling surface greatly influenced the cooling performance by 41.4%. The cooling water flow rate, heat exchange effectiveness of the water block, and heat resistance of the hot side were confirmed to have relatively little influence as 9.7–10.2%, 11.9–24.8%, and 0.7–11.1%, respectively.

Influencing Factors of a Cooling System Based on Low-Temperature Mine Water as a Direct Cooling Source

With the depletion of shallow metal mineral resources, deep mining has become more common. In the process of deep mining, heat hazards in mines seriously threaten the health of personnel and the safety of mining operations. According to the flow and low-temperature characteristics of abundant water in Maoping Lead Zinc Mine, this paper proposes the direct use of the low-temperature water inflow of the mine as the cold source by which to conduct the heat exchange with a single spiral-tube heat exchanger for mine cooling. An experimental platform was built for the cooling system to allow us to explore the mechanism of the influence of the inlet air volume and air temperature and the inlet water temperature and flow on the cooling effect of the single spiral-tube heat exchanger. It was found that with the increase in the inlet air volume and inlet water temperature, the cooling efficiency of the heat exchanger decreased, while with the increase in the inlet air temperature, the cooling efficiency of the heat exchanger increased. Within the experimental range, 127.2 m3/h was found to be the optimum inlet air volume, and 33.0 °C was the most appropriate inlet air temperature; the water temperature of Maoping Coal Mine is about 20 °C throughout the year, and the industrial test site can reduce the wind temperature of 31.5 °C to 23.9 °C. The inlet water flow is positively related to the cooling effect. With the increase in the water flow, the outlet temperature of the air flow at each working point was continuously reduced, and the cooling effect of the heat exchanger was improved. The moisture content of the inlet air flow can be reduced by increasing the low-temperature inlet water flow. Through experiments, the feasibility of the cooling system that directly uses the mine’s low-temperature water as the cold source was verified. A multiple linear regression analysis equation for the cooling system model is proposed, which provides a reference for formulating effective measures to prevent and control heat hazards in mines.

Experimental study on effectiveness in segmental baffled shell and tube heat exchanger using Al2O3 nanofluid at variable concentrations

Nanoparticles attracted many researchers because of their significant heat transfer enhancement properties in various applications. The nanoparticles have variable properties and a high specific surface (Cp) area, which enhance heat exchange between fluids and their particles at different volume concentrations. This article provides the outcomes of an experimental study on the overall heat transfer coefficient (Uo) and effectiveness ( ε) of the shell and tube heat exchanger (STHE) with a 25% baffle cut using Al2O3 nanofluid consisting of base fluid distilled water (DW) and Al2O3 nanoparticles at 0.05%, 0.1%, and 0.2% of volume concentrations. Using a two-step method and adding a weight fraction of 0.1% sodium dodecylbenzene sulfonate (SDBS) as a surface active agent to each volume concentration of nanofluid provides better stability. The thermophysical properties like density (ρ), thermal conductivity (k), and dynamic viscosity ( μ) of the Al2O3-DW nanofluid increased, but Cp of the same was reduced by increasing the volume fraction. Al2O3-DW nanofluid at different concentrations in parallel and counter flow variations of the STHE enhances the Uo and ε when compared with water. However, counterflow provides higher effectiveness. At 0.6 LPM of the nanofluid flow and hot fluid at T = 80°C temperature, the difference in the heat exchanger's effectiveness enhancement is 4.24% for parallel flow and 4.16% for counterflow at 0.2% concentration of Al2O3-DW nanofluid compared to water.