Heat Exchange Efficiency Research Articles

Pumped heat energy storage with liquid media: Thermodynamic assessment by a transcritical Rankine-like model

A pumped heat energy storage (PHES) system based on a Rankine cycle for supercritical working fluids, such as carbon dioxide and ammonia, accounting for the irreversible latent and sensible heat transfers between the working fluid and the storage liquid medium, as water or thermal oil, is analyzed. The model also includes several parameters such as pressure losses, heat exchanger efficiencies, and isentropic efficiencies of the compressor, pump, and expansion devices (such as turbines and valves), that take into account the main internal and external losses and heat leak to the environment. The model allows for the calculation of specific energy, the heat pump performance coefficient, heat engine efficiency, and overall round-trip efficiency, as well as the temperatures of the working fluid and reservoirs. A zero-dimensional model is also used to determine the time dependence of heat leak in the tanks. The main results show that this technology could achieve round trip efficiency values in the order of 50–70%. Irreversibilities in compression and expansion appears as the most influential energy losses factor. The time effect of the ambient conditions on the tanks has been analyzed for a wet subtropical climate but it seems that the ambient conditions have no major influence on the performance of the system. In addition, explicit numerical results and temperature–entropy plots are presented for two representative systems as carbon dioxide-water and ammonia-thermal oil to take into account the main values in an operating condition.

Numerical investigation of subcooled flow boiling in an inclined rectangular mini-channel at a low flow rate

Under a low flow rate, gravity may become prominent for bubble behavior and heat transfer of flowing boiling because of the weakness of drag force from liquid, and its effect changes with the inclination angle of the mini-channel but without consensus. In this paper, based on a reasonable nucleus site density model and considering conjugate heat transfer, the coupled volume-of-fluid and level set method is adopted to study the subcooled flow boiling in an inclined three-dimensional rectangular mini-channel (0°–180°) with a characteristic size of 1.0 mm at a low flow rate of 88.8 kg m−2 s−1. The inclination angle is found to have a slight effect on the flow boiling, which is different from the conclusion drawn based on the traditional-macro channel. A bubbly flow appears when a heat flux of 300 kW/m2 is added. An unconventional impact force is proposed, which presses large bubbles to slip along the heating wall, with slight differences in the flow pattern under different inclination angles. When the inclination angle is close to 0°, the upstream small/medium bubbles leave the heating wall under gravitational effects, which is conducive to heat transfer. As the inclination angle approaches 90°, gravity pushes the large bubbles downstream to leave the channel, favoring the rewetting of the dry patches below. These two positive effects fail as the inclination angle approaches 180°, leading to slightly worse overall heat exchange efficiency. However, the maximum differences in the average and local wall superheating of the mini-channel are only 8.4% and 22.5%, respectively, across the range of inclination angles because the flow pattern remains similar under the effect of the impact force. In addition, the effect of inclination angle on flow boiling becomes weaker with the increase in heat flux because of happening of slug flow.

Study on the thermal storage performance of a new electric heating device with phase change materials

The heating method for reducing the viscosity of crude oil is mainly electric heating currently. In order to meet the needs of environmental protection and industrial production, a new electric heating device with phase change thermal storage is designed by combining the crude oil viscosity reduction heating method, off-peak electricity, and phase change materials. Numerical simulation research is carried out for the phase change thermal storage and release performance of the device, and the structure is optimized according to the research results. Also, the influences of different inlet flow rates and metal foams porosity on the heat release process are analyzed. The results show that the outlet temperature decreases continuously with the increase of the working medium inlet flow rate. With the addition of nickel foam, the internal temperature of PCMs becomes more uniform as the porosity of the added metal foam decreases in the range of 0.7–1.0. Using this device to reduce the viscosity of crude oil cannot only maintain high heat exchange efficiency but also reduce production costs, which provides a reference for the design and operation of crude oil viscosity reduction methods.

Investigation of thermal performance and entropy generation in a helical heat exchanger with multiple rib profiles using Al2O3-water nanofluid

This research aims to assess the in detail thermal performance and entropy generation of a helical heat exchanger with multiple rib profiles and coil revolutions using water-based Al2O3 nanofluid with 5% concentration. A steady-state computational fluid dynamic model was used in determining the thermal and hydraulic parameters. The numerical model was validated with a numerical study and an experimental study. Three multiple rib profiles (2 rib, 3 rib and 4 rib) and three different coil revolutions (10, 20 and 30) were considered to design nine cases of heat helical exchangers. The geometrical effect was assessed and further represented as the streamlines, isotherms, overall Nusselt number, friction factor, thermal enhancement factor, and entropy generation. It is found that with the growth of coil revolutions the overall heat transfer rate and friction factor rise. The most efficient heat exchanger found in terms of thermal enhancement factor is 3 rib 10 revolutions with the value of 1.34. The entropy generation increases with the rise of the coil revolution. The maximum entropy generation increased by 19.5% for varying the coil revolution with a constant rib profile. Finally, this study is a guide of choosing an efficient heat transfer in terms of thermo-hydraulic performance.

Fluid flow and heat transfer characteristics of corona wind in cross-cut finned channel

An efficient heat exchanger should exhibit superior heat transfer performance with low pressure drop. However, in the case of forced convection such as fan cooling, the heat transfer performance is proportional to the area of the extended surface and the airflow rate, which accompanies a pressure drop and imposes a limit on performance improvement. Herein, flow acceleration using corona wind is proposed as a method to simultaneously solve the problem of pressure drop in heat exchangers and improve heat transfer performance. Further, the concept of accelerating the flow of local regions around the surface by installing a wire electrode on the fins of a heat exchanger for corona wind generation is implemented. In addition, an original form of heat exchange fins that combine wire electrodes and cross-cut fins is presented. By controlling the flow velocity direction of the corona wind in the downstream direction of the mean stream, the pressure drop is suppressed and maximize the heat-transfer performance. Detailed experimental data on the velocity and temperature of the corona wind from the wire to the cross-cut fins with various wire materials, positions, and dimensionless design parameters in the channel are presented. In addition, the flow patterns of the accelerated corona wind are analyzed using particle image velocimetry on the cross-cut fins. The results confirm that the corona wind can be applied to a cross-cut fin array and can significantly enhance the heat-transfer performance of the fins.

Two-phase simulation of a shell and tube heat exchanger filled with hybrid nanofluid

The major purpose of the present research study is to design an efficient heat exchanger filled with hybrid nanofluid numerically by making use Al2O3-Cu-water hybrid nanofluid, FVM and SIMPLEC algorithm. The influences of utilizing ribbed tube and then applying nanofluid are probed. Based on obtained findings usage of inner grooves for tube has an obvious effect on the rise of heat exchanger. The inner grooved heat exchanger with N = 3, P = 12 mm, a = 0.9 mm and q = 50° is introduced as the most efficient inner grooved model filled with nanofluid. Further, it is shown that utilizing nanofluid improves the heat exchanger's energy efficiency. Also one of the important goals of this paper was to choose the optimum grid-mesh (GM) along with the least calculation time as well as the highest accuracy. Due to fulfill this demand, four diverse grid-meshes models were developed. For each of the models, various grids were developed and then checked for the analysis of error percent as well as estimation of the time values. The GM-Four with 930289 nodes is considered to ensure a good agreement in the 137 min. The model with Re=2,000,000 and φCu = 0.10 & φAl2O3 = 0.05 at has the most efficiency among all models.

High shear mixer works as a heat exchanger enhancing the liquid–liquid direct contact heat transfer

As an indispensable unit equipment in industrial production, the design of efficient heat exchangers is of great significance to improve energy utilization rate. In this paper, the in-line high shear mixer (HSM) was investigated as a heat exchanger for the first time by experiments and assisted with CFD simulations, and the heat transfer performance was measured using kerosene and thermal water as the heat transfer fluids. Effects of operational parameters, including rotor speed, fluids temperature and flow rate, as well as the structural parameters, involving the rotor teeth number, teeth height, and the shear gap, were studied on the volume heat transfer coefficient and the heat transfer efficiency of such inline HSM. The results indicated that the volume heat transfer coefficient can be significantly improved by increasing the rotor speed and the flow rate of the heat transfer fluids, but reducing the heat transfer temperature difference, whereas increasing the thermal fluid flow rate deteriorates heat transfer efficiency. Furthermore, the structural parameters including rotor teeth number and teeth height, the shear gap within the stator and rotor were optimized to enhance the heat transfer of HSM. The optimal HSM possesses a shear gap of 0.5 mm, a teeth height of 10 mm and a teeth number of 6, which can generate larger turbulent energy dissipation rate and higher shear rate, meanwhile make more fluids jet out of the shear gap and stator hole. The maximum volume heat transfer coefficient and heat transfer efficiency can be up to 2522.8 kW·m−3·°C−1 and 92.4% with the optimal structure. Finally, the dimensionless correlations of heat transfer performance were established to guide the operating mode and structural design of high-efficiency high shear mixer.

Durable and robust superhydrophobic fluororubber surface fabricated by template method with exceptional thermostability and mechanical stability

Superhydrophobic surfaces have a wide application prospect in daily life and industrial fields. However, these surfaces are vulnerable to various thermal and mechanical damages during service and lose their functions. Herein, a superhydrophobic fluororubber surface with orderly layered microprotrusion structure was successfully prepared by directly replicating the surface microstructure of stainless-steel mesh. The prepared superhydrophobic surface exhibited excellent superhydrophobic performance (153.93 ± 0.39°) and durability. Due to the excellent thermal stability of fluororubber, the surface can maintain its superhydrophobicity even heated, and can also endure continuous high-temperature exposure. Especially, the superhydrophobicity of the surface does not change when contacting high-temperature droplets. In addition, as a result of the direct formation of microstructure on the surface, the superhydrophobic fluororubber surface also shows excellent robustness to manual friction, sandpaper abrasion and cyclic bending and twisting. The superhydrophobic fluororubber surface has great potential of application in the fields of efficient heat exchange, seawater desalination and other.

Characteristic simulation and working parameter optimization of the aggregate dryer

The aggregate dryer is generally used in an asphalt mixing plant for road construction equipment and serves to heat the aggregates. In this work, the coupled method of Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) is used to study the gas-solid coupling of aggregate dryers to improve the heat exchange efficiency between flue gas and aggregate. The heating process of aggregates in an aggregate dryer is visualized by analyzing the particle discrete field and the fluid temperature field during drying. The motion process and distribution characteristics of aggregate particles are established for different outputs and rotational speeds, and the hot flue gas distribution and temperature characteristics of the aggregate dryer are also obtained. The results show that the rotational speed affects the distribution of the aggregate veiling zone and pressure, which in turn affects the stability of the flame. Reducing the rotational speed results in a more uniform distribution of the aggregate veiling zone and thus an increase in heat utilization. In addition, when the output of the aggregate dryer is 80 t/h and the rotation speed is 6.2 rpm, the aggregate discharge temperature is the highest and the thermal efficiency of the aggregate dryer is the best.

Comprehensive review of spiral heat exchanger for diverse applications

Energy-saving is a very important issue in our world and heat exchanger is very useful for saving energy. Heat exchanger selection is very important in process industries like dissolution, fermentation, crystallization, etc. Spiral Heat Exchangers (SHE) are known for their compact design and excellent heat transfer efficiency when compared to their conventional counterparts. The surface area utilized for heat transfer is much larger in a spiral heat exchanger as compared to conventional designs like shell-type heat exchangers for the same volume occupied. These advantages make SHEs much more suitable for a variety of applications in processing industries. This present article focuses on the research and optimization done by various researchers across the world to improve the efficiency of spiral heat exchangers by different approaches.

Thermal Efficiency of Involute Heat Exchangers

Thermal Efficiency of Involute Heat Exchangers

Study on the influence of casing heat transfer enhancement

Heat transfer is one of the most common unit operations in industrial production. In this paper, the variation law of heat transfer coefficient of smooth tube heat exchanger and tube heat exchanger with spiral coil at different flow rate is discussed by experiment and numerical simulation, and the corresponding correlation formula of convective heat transfer criterion is fitted. This has certain reference significance to improve the heat transfer efficiency of heat exchanger to reduce the energy consumption of the country and improve the design of heat exchanger structure.

Research on the influence of secondary heat network characteristics on wind power consumption in the combined electric and heat system

With the large-scale investment of thermal power units in the “Three Norths” area, the coupling of the combined electric and heat system has become more and more close, leading to a huge number of abandoned wind. On the basis of the traditional thermal system model, physical characteristics of the secondary heat network are added, and an optimal scheduling model of the combined electric and heat system considering the operation characteristics of the secondary heat network is established. The influence of the operation characteristics of the secondary heat network on the heat loss and wind power consumption of the thermal system is studied. Based on this, different conditions are set up to study the influence of heat-exchange efficiency of heat exchange stations and the diameter of secondary heat network pipes on the combined electric and heat system. The simulation analysis shows that the heat network characteristics of the secondary heat network need to be considered in the scheduling analysis of the combined electric and heat system. In addition, the system air curtailment rate can be reduced by improving the heat exchange efficiency of the heat-exchange station and changing the diameter of the secondary heat network pipeline.

Ultra-low hysteresis in giant magnetocaloric Mn1-xVxFe0.95(P,Si,B) compounds

Large thermal hysteresis in the (Mn,Fe)2(P,Si) system hinders an efficient heat exchange and thus limits the magnetocaloric applications. Substitution of manganese by vanadium in the Mn1-x1Vx1Fe0.95P0.593Si0.33B0.077 and Mn1-x2Vx2Fe0.95P0.563Si0.36B0.077 compounds enable a significant reduction in the thermal hysteresis without losing the giant magnetocaloric effect. For the composition closest to the critical one, where first-order crossovers to second-order phase transition in the series of x2 = 0.02, Mn0.98V0.02Fe0.95P0.563Si0.36B0.077 exhibits a thermal hysteresis that is reduced from 1.5 to 0.5 K by 67%, yielding an adiabatic temperature change of 2.3 K and magnetic entropy change of 5.6 J/kgK for an applied field of 1 T, which demonstrates its potential for highly efficient magnetic heat pumps utilizing low-cost permanent magnets.

Experimental study on heat exchange efficiency of rock bed heat storage system based on broken rock mass

Energy storage system can significantly improve energy utilization by ”cutting peak and filling valley” for the development and use of renewable energy sources such as wind and solar energy. Although rock packed bed is one of the main forms of sensible heat storage, understanding the variation law of its permeability and heat exchange efficiency is critical to the construction of large-scale heat storage systems. In this paper, we used CO2 as the heat transfer fluid in a self-built experimental equipment to conduct heat injection and extraction experiments on broken rock packed beds with different grain compositions and injection pressures, and we studied the heat exchange efficiency and the variation law of heat storage in the rock bed during the heat injection and extraction process. The results show that: (1) with the heat injection, the permeability of the rock bed filling system decreases gradually; and (2) the rock grain composition has little effect on heat exchange efficiency but a large effect on permeability. The higher the permeability, the greater the heat storage of rock bed; (3) The higher the fluid injection pressure, the greater the heat exchange efficiency and the greater the heat storage of the rock bed. The research results can provide guidance for the economic operation of broken rock bed heat storage system.

Novel fabrication of honeycomb-like magnetocaloric regenerators via a self-organization process

The geometry of a magnetocaloric regenerator (MR) is crucial to the heat exchange efficiency and the performance of a magnetic refrigerator. Here we proposed a new and facile way to produce monolithic La0.67Ca0.33MnO3 MRs with highly ordered and unidirectional oriented microchannels. This is realized by a self-organization process of alginate sol, which does not require specific processing equipment and hence is well-suited for large-scale production. The average diameter and total porosity of the well-defined microchannels can be adjusted in the range of 71.5–156.3 μm and 15.3%-21.5%, respectively. The orthorhombic perovskite structure is retained in the sintered MRs without any detectable secondary phase. Besides, the sintered MRs exhibit a significantly higher isothermal entropy change than the original La0.67Ca0.33MnO3 powders due to an increase in the grain size. Consequently, our study may trigger a breakthrough in the large-scale production of monolithic MRs with desirable geometries for magnetic refrigerators.

Modeling and estimation of fouling factor on the hot wire probe by smart paradigms

The fouling factor is a complex non-linear function of several feature variables. Accurate estimation of this measurable factor is applicable for controlling the heat transfer efficiency of heat exchangers. Artificial neural network (ANN) and adaptive neural-based fuzzy inference system (ANFIS) techniques are accurate approaches for the understanding treatment of the most complicated systems. The cascade-feedforward (CFF), multi-layer perceptron (MLP), radial basis function (RBF), and generalized regression (GR) neural networks and ANFIS were applied for predicting the fouling factor of the hot wire probe from some measured variables using a huge databank, including 1870 empirical dataset. Pearson's and Spearman's techniques confirmed the highest relation between considered features and the first order of fouling factor. The results demonstrate that the cascade feed-forward neural network containing eight hidden neurons was the best artificial intelligence (AI) model with excellent overall AARE = 3.44 %, MSD = 0.0000315, RMSD = 0.0056, and R2 = 0.9982. This work's significance lies in presenting an applicable and accurate tool for estimating fouling factors on hot wire probes to the research community and industry. This model can be considered a reliable replacement for empirical analyses that are often expensive and time-consuming.

Thermal flow analysis of self-driven temperature-sensitive magnetic fluid around two cylinders arranged in tandem

A temperature-sensitive magnetic fluid (TSMF) is a refrigerant that provides high heat transfer capability and pump-less long-distance heat transfer. The driving force of the self-driving TSMF is produced by the temperature difference and the magnetic field gradient. Due to this principle, expanding the heating area can improve the driving force while promoting the heat exchange efficiency between the heating body and the TSMF. This study focuses on heat flow phenomena and heat transfer around two cylinders arranged in tandem. We used the hybrid Boltzmann method to analyze the thermal flow of a self-driven TSMF and investigate the effect of distance between the two cylinders and the magnetic field magnitude of the heat transfer. The results showed that, when the magnitude of the magnetic field is small, the heat between the cylinders is not easily dissipated, which causes fluctuations in the driving force. The heat flow from the upstream cylinder has a direct effect on the downstream cylinder. A variation of the distance between the cylinders produces two different heat flow phenomena, the first type of heat flow phenomenon is characterized by the movement of heat along the central axis and the second type is the movement of heat from the trailing edge of the upstream cylinder to the trailing edge of the downstream cylinder. Compared with the first type of heat flow phenomenon, the heat transfer effect of the second type of phenomenon is significantly improved.

Effect of hydrocarbon condensation on fouling and heat exchange efficiency in EGR cooler

Based on the self-established experiment and test platform and evaluation system, we studied the effects of hydrocarbon condensation on mass, hydrophilicity, color, and micromorphology of fouling and heat transfer efficiency in exhaust gas recirculation (EGR) coolers. The results show that hydrocarbon condensation converts low mass, hydrophobic, solid black, and fluffy scales into larger mass, hydrophilic, white and yellow, and compact scales of fouling. When Total Hydrocarbons (THC) are 590 ppmC, 411 ppmC, and 359 ppmC, the temperatures to the demarcation points of the rapid condensation enhancement and stability stage are 100 °C, 120 °C, and 135 °C, respectively. However, when the temperature exceeds 160 °C, hydrocarbons' condensation is substantially suppressed, and the heat exchange efficiency is the worst. Hydrocarbons condensation will increase the fouling density, improving the heat exchange efficiency; the highest heat exchange efficiency will be achieved when the condensation is stable. Hydrocarbon and particulate matter (PM) together determine the composition and structure of the fouling, which in turn determines the heat transfer efficiency.

Heat recovery ventilation design limitations due to LHC for different ventilation strategies in ZEB

Today's buildings are becoming more insulated and airtight to reduce transmission heat losses. Energy use for ventilation can represent up to half of these buildings' total energy use. Heat recovery in ventilation and demand-controlled ventilation (DCV) are energy-efficient measures to reduce ventilation energy use, especially when combined. However, this study revealed that the often-overlooked longitudinal heat conduction (LHC) in aluminium rotary heat exchangers might yield less efficient heat exchangers, particularly for intended high-efficiency heat recovery at low ventilation rates in DCV. This study presents a theoretical method to assess the effect of LHC on the amount of energy used to heat ventilation air for several ventilation strategies. The method is demonstrated in a case study for a virtual office building in a cold climate (Oslo, Norway). When neglecting the LHC effect, the energy used to heat the supplied air using DCV with a rotary heat exchanger is about three times smaller than when considering LHC. Unlike earlier studies, we find that DCV may consume more ventilation heating energy than constant air volume (CAV) ventilation when the selected wheel is deep and oversized due to LHC. This study highlights the need to design rotary heat exchangers carefully in order to account for the LHC effect, particularly when targeting zero emission buildings (ZEB).