Heat Exchange Efficiency Research Articles

The Numerical Simulation of Thermal Efficiency of Triple Pipe Heat Exchanger Using PCMs (Paraffin and Lauric Acid) System

COMSOL Multiphysics is used to adequately study the numerical modeling of phase change material behavior in a triple heat exchanger. The triple heat exchanger comprises three sides: an interior pipe for water cooling and heating, an annular space between the second and first interior pipes conveying Paraffin, and a third side annular space between the outer pipe and the second pipe holding Lauric acid. The dimensions of the triple heat exchanger are as follows: 30 cm length and side diameters of (0.5 in: 2.5 in: 3.5 in). Overall, the patterns reveal that the CFD and experimental work are in perfect agreement. PCM 2 is more reliable than paraffin. As the flow rate rises, the validity decreases; the maximum validation is obtained at 11 L/min. The results shows that the number of fins enhances efficiency by 5% (from 6 to 4); any number of fins over 6 promotes no more efficiency percentage. The additional fins are considered to produce increased heat transfer resistance, resulting in a significant increase in heat transmission. Copper slightly conducts heat better than aluminum and iron. Aluminum is the best material; its efficiency is equivalent to copper's. The diameter of the pipe has no effect on the percentage of paraffin efficiency. The better the heat resistance with an uniform heat transfer area between the paraffin and lauric acid layers, the broader the diameter of the lauric acid layer. Improving the outer diameter of the heat charge process implies increasing the efficiency percentage. The highest efficiency% is seen at 4.5 in.

Polysiloxane-Based Liquid-like Layers for Reducing Polymer and Wax Fouling

Surface fouling occurs when undesired matter adheres and accumulates on a surface, resulting in a decrease or loss of functionality. Polymer and wax fouling can cause costly blockages to crude oil pipelines, clog jet fuel injectors, foul chemical reaction vessels, and significantly decrease the efficiency of heat exchangers. Fouling occurs in many forms but can be segmented based on adherent size, modulus, and chemical functionality. Depending on the foulant, surface design strategies can vary greatly. Few strategies exist to prevent the buildup of wax and polymers on surfaces. In this report, we investigate the potential of highly disordered, siloxane liquid-like layers as a strategy for reducing wax and polymer deposition. In our tests, it was found that the liquid-like layers developed here were able to reduce postadsorption roughness for polymer and wax by as much as 35- and 47-fold, respectively, when compared to the control. SFG was utilized to investigate the molecular-level interfacial properties for each of the modified surfaces to help understand the antifouling mechanism. The data showed that the likely higher grafting density and a large degree of random conformational freedom at the liquid-surface interface make the developed siloxane-covered surfaces energetically unfavorable for polymer and wax accretion.

A Numerical Analysis of the Thermal Energy Storage Based on Porous Gyroid Structure Filled with Sodium Acetate Trihydrate

The present paper deals with the evaluation of the unique design of the thermal energy storage unit and its impact on the overall heat exchange efficiency. The proposed thermal energy storage unit consists of a gyroid thermally conductive structure, the voids of which are filled with sodium acetate trihydrate. The presented concept is focused on the use in the field of heavy machinery, where it is possible to accumulate and re-use waste heat from internal combustion engines from the cooling liquid or lubricating products. The evaluation of designs took place through numerical simulations on three models characterized by different levels of the introduction of the gyroid structure into the design. From the design point of view, the gyroid structure was considered as an object produced by additive manufacturing methods from a thermally conductive filament based on a thermoplastic polymer, which enables considerable simplification of production compared to the use of suitable anti-corrosion metals. A comparison of the essential thermophysical parameters in the process of charging and discharging of the proposed thermal energy storage unit quantified a significant increase in the rate of the charging, respectively, of the discharging process, manifested by a rapid increase in the temperature of the sodium acetate trihydrate volume, respectively, of the output temperature of the heat transfer medium that removes the accumulated heat for further use.

Numerical Analysis of Flow and Heat Transfer Characteristics of Lattice-Based Compact Heat Sinks

Abstract Single unit cell thick lattice frame materials have applications in efficient heat exchangers. This study is focused on strut-based sandwich-type configurations obtained through reticulation of unit cell topologies of tetrakaidecahedron (TKD), octet, and rhombic dodecahedron (DDC) shapes at a porosity of 0.9 with water as the working fluid. Interfacial heat transfer coefficient values on struts and endwalls were determined by imposing constant temperature boundary condition. Averaged heat transfer coefficient on the endwall was the highest for tetrakaidecahedron lattice whereas rhombic dodecahedron lattice exhibited the highest average interfacial heat transfer coefficients on the struts. Flow analysis showed the presence of strong secondary flow features on planes normal to the mean flow direction that demonstrated the unique flow mixing capabilities of these lattices. Reported interfacial heat transfer coefficient at struts and endwall can be used in volume-averaged computations of metal foams (representative of lattices' flow and thermal properties) under local thermal nonequilibrium.

Experimental study of adsorption CO2 storage device for compressed CO2 energy storage system

Compressed CO2 energy storage is a reliable physical energy storage solution. The main challenge of compressed CO2 energy storage system is how to solve the high-density storage of low-pressure CO2. In this study, we proposed a new type of adsorption transcritical compressed CO2 energy storage system. We used adsorbents to adsorb CO2 for achieving low-pressure, high-density storage of low-pressure CO2. We investigated the gas storage capacity of the adsorption storage system and the heat and mass transfer process. Results demonstrate that the storage density (57.6 °C–25 °C) of adsorption gas storage is 43.46 kg/m3, which is 24.8 times of the CO2 density (1.75 kg/m3, 30 °C, 1 bar). If the adsorbents are heated above 160 °C, then the storage density will increase 3–4 times. The whole process is controlled by heat exchange efficiency. The heat exchange efficiency greatly affects the response time of the system during the application.

Intensification of thermal efficiency of a cross-flow heat exchanger under turbulent flow conditions using CuFe2O4/water nanofluid

The use of nanofluids has opened new intellectual horizons in improving the performance of heat exchangers. Therefore, in this work an experimental study was conducted for investigating the effect of CuFe2O4/water nanofluid on the heat transfer rate enhancement in a cross-flow type heat exchanger under turbulent flow conditions. Heat transfer rate were examined at different volume percentages of CuFe2O4 nanoparticle (0≤ φ ≤ 0.9% vol.) with a nominal diameter of 28 nm at three inlet fluid temperatures of 72, 80 and 88 °C, water pump rotational speeds of 2000 rpm and 3000 rpm, and cross-flow fan speeds of 1500, 2000 and 2500 rpm. According to the results, the heat transfer rate in the case of using nanofluids with a volume percentage of 0.1% was significantly higher than that of distilled water in all test conditions. The highest relative increase in the universal heat transfer coefficient (U) equal to 92.3%, compared to the use of water (φ = 0) was obtained at 0.1% vol% of nanoparticles, the lowest temperature of fluid entering the heat exchanger (72 °C), medium pump speed (2000 rpm) and low fan speed (1500 rpm). The maximum Performance Evaluation Criterion (PEC) improvement of 44.46% also occurred at this working condition.

A Critical Review on Geometric Improvements for Heat Transfer Augmentation of Microchannels

With the application of microdevices in the building engineering, aerospace industry, electronic devices, nuclear energy, and so on, the dissipation of high heat flux has become an urgent problem to be solved. Microchannel heat sinks have become an effective means of thermal management for microdevices and enhancements for equipment due to their higher heat transfer and small scale. However, because of the increasing requirements of microdevices for thermal load and temperature control and energy savings, high efficiency heat exchangers, especially microchannels are receiving more and more attention. To further improve the performance of microchannels, optimizing the channel geometry has become a very important passive technology to effectively enhance the heat transfer of the microchannel heat sink. Therefore, in this paper, the microchannel geometry characteristics of previous studies are reviewed, classified and summarized. The review is mainly focused on microchannel geometry features and structural design to strengthen the effect of heat transfer and pressure drop. In addition, the correlation between boiling heat transfer and geometric characteristics of microchannel flow is also presented, and the future research direction of microchannel geometry design is discussed.

A new practical approach to design a Phase Change Material-Water longitudinally finned tube heat exchanger

Phase change materials have demonstrated interesting advantages in solar energy storage systems to produce domestic hot water although more efficient heat exchangers are required. Among the different exchanger typologies, longitudinally finned exchangers are widespread. However, according to scientific literature, there is a lack of analytical models able to estimate the thermal power transferred by this kind of exchangers easily. This paper shows a practical and easy to apply analytical model that has been built for that type of exchangers. Thus, based on a change of the solution given by Heisler and Gröber for an infinitely long cylinder, this new analytical approach characterises the thermal energy discharged in terms of dimensionless numbers. The adjustment of the analytical model was achieved from the results provided by a CFD numerical model which was previously validated from experimental data. 544 numerical simulations of the system were carried out varying the Biot number and a surface factor number from 1 to 100 and 3 to 15, respectively. An analytical equation that estimates the energy discharged is provided as a function of Fourier, Biot, and Stefan numbers and the surface factor number, related to the exchanger geometry. An average value of the root mean square error smaller than 4% indicates that the proposed model is sufficiently accurate and might be used in the design stages of this kind of devices. A further validation of the analytical model against experimental observations will be carried out in a future stage of model development.

Thermo-Hydraulic Performance of Heated Tube with Twisted Delta Winglet Swirler Insert: A Numerical Simulation

Among the various methods for enhancing heat transfer in a heat exchanger, a passive method of inserting a continuous swirler inside a heated tube provides a secondary flow along the fluid that reduces the thickness of the thermal boundary layer, thus increasing the efficiency of convection heat transfer performance. The research’s primary goal is to conserve energy, materials, and money by operating efficient heat exchanger equipment. However, the continuous swirler along the fluid flow creates a persistent obstruction, which amplifies the friction factor and increases the working fluid’s energy loss. As a result, this research presented the twisted delta winglet swirler (TDWS), a new design of a decaying swirler that uses delta winglets twisted to 180° to produce a swirling flow along the tube. The swirler comprises four twisted delta winglets arranged in a circle with a diameter 6% smaller than the tube and a length of L/D=2.2. It was placed at the entrance to a heated tube test section with a diameter of 0.016 m and a length of L/D=93.75. The Reynolds Stress Model was used to simulate the flow domain with a water-ethylene glycol mixture was chosen as the working fluid. TDWS transformed the uniform inlet flow from potential energy to high kinetic energy, resulting in a high intensity of swirling flow downstream of the circular tube up to L/D=46.88 before decaying and reaching a steady state. Compared to other decaying swirlers, TDWS obtained one of the lowest relative friction factors, 1.36, with this flow. The maximum global relative Nusselt number increased by only 11% because this value considered the area where the flow reached a steady state. Since the TDWS is a decaying swirler, the thermal-hydraulic performance reached unity along the tube. However, the optimal performance of TDWS in the plain tube with a length of L/D=93.75 can be found if the dimension or geometric configuration of the TDWS is modified, or two or more TDWS may be placed in an array orientation.

Study of the effect of the annular gap between the kta housing and the transverse segment partitions on the heat exchange efficiency

In modern industry, specially designed devices, called heat exchangers, are used to carry out heat exchange between working bodies (heat exchangers). They are widely used in all industries (chemical, oil and gas, food, etc.), which is due to the high reliability of the design, high productivity, simplicity of design and a variety of types and designs. The objective of the investigation is to simulate heat exchange in a shell-and-tube heat exchanger without an annular gap between the housing and segment transverse partitions and with a gap equal to 6 mm, and then determine at which of these two gaps the heat exchanger thermal efficiency is highest. The simulation was performed in ANSYS Workbench. The comparison of the energy efficiency of the apparatus was made using the Kirpichev criterion. This criterion is quite simple and does not require finding a large number of various parameters. With this criterion, it is possible to determine in which case the heat exchanger will be most advantageous in terms of energy efficiency.

In-situ fabrication of superhydrophobic surface on copper with excellent anti-icing and anti-corrosion properties

The accumulation of ice on electrical construction works not only leads to the reduced efficiency of power supply and heat exchangers, but also increases the energy losses and even brings about serious disasters. Therefore, it is essential to reduce or eliminate the issue of ice accumulation on metal constructions. Nowadays, superhydrophobic surfaces have received lots of attentions for their outstanding hydrophobicity. In this study, Cu(OH)2 nano grass was firstly in-situ constructed on copper surface via simple anodization, then superhydrophobic CuO nano needles were obtained after the dehydration of Cu(OH)2 and modification with fluoro-alkyl silanes. The results showed that when the anodic oxidation temperature was 20 °C and the current density was 2 mA·cm−2, nano-needle-like multistage micro-nano structures can be obtained. The as-prepared superhydrophobic surface showed the water contact angle as large as 162° and sliding angle lower to 4°. Additionally, the as-prepared superhydrophobic surface displayed outstanding anti-icing property, delaying icing substantially even under − 10 °C environment regardless of droplet volume and type. Moreover, it also revealed good anti-corrosion property with a corrosion current density of 2.2 × 10−8 Ω·cm2 and the corrosion resistance of 1.55 × 106 Ω·cm2 which is about three order of magnitudes than that of bare copper. We prospect that this easily prepared surface can provide a solution of anti-icing and anti-corrosion for electrical construction works and large-scale metals.

Experimental Analysis of Two Different Heating Systems Using Copper Pipe and Carbon Steel Pipe in Shell and Tube Heat Exchanger

In this thesis, the experimental analysis of two different systems was carried out by using copper tube and steel tube in shell and tube heat exchangers. The experimental systems were designed and manufactured to have the same physical properties in two types of heat exchangers. Manufacturing costs were noted to enable comparison. 
 
 In the system where the experiments were carried out, only the heat exchanger bodies were replaced and the analyses were carried out and the other systems were kept constant. In order to perform an efficient analysis of the efficiency, the inlet temperature of the heater water was kept constant and it was not exposed to any recirculation or heat loss. Although it was observed that the efficiency of the copper tube heat exchanger increased significantly when the inlet water temperature was high during the experiments, there was no significant difference in efficiency between the two heat exchangers at low temperatures.
 
 As a result of these analyses, it has been concluded that if the heat transfer is to be carried out for low temperatures, considering the cost, the carbon steel tube heat exchanger can be used despite of the high thermal conductivity of copper.

Structure effect on thermal-hydraulic performance in liquid metal – Aviation kerosene printed circuit heat exchanger

A liquid metal to kerosene printed circuit heat exchanger (PCHE) is initially presented in this study to address the heat transfer efficiency and compactness difficulties of traditional heat exchangers aboard aircraft with increasing heat flux. The structure of PCHE must be particularly constructed because of the extremely high heat conductivity and low Prandtl of liquid metal. As a result, for the structural design and optimization of PCHE in this study, three-dimensional numerical simulation method is used, and an experimental platform for Ga-In-Sn alloy and aviation kerosene using a spiral heat exchanger is built to verify the precision of simulation results. The optimal characteristics of a PCHE microchannel based on Ga-In-Sn alloy are established, according to simulation results: a trapezoidal cross-section, 0° of inclination, and a single hot side-double cold side structure. The flow and heat transfer characteristics in the axial direction of the TT2 microchannel demonstrate that temperature has the minimal effect on the convective heat transfer of Ga-In-Sn alloy under low Re conditions. However, temperature improves the convective heat transfer of Ga-In-Sn alloy in high Re working circumstances, suggesting that Ga-In-Sn alloy is better suited to cooling situations with high heat flux and hypersonic aircraft cooling.

Seasonal energy efficiency ratio of regenerative indirect evaporative coolers—Simplified calculation method

Evaporative-cooling technology is a promising alternative to conventional air-cooling systems that are based on using direct expansion units in terms of energy efficiency. Seasonal energy efficiency ratio (SEER) calculation methodologies and standards for these air-cooling systems remain limited. Therefore, the objective of this study is to develop a novel simplified method for calculating the SEER for regenerative indirect evaporative coolers (RIEC). Experimental tests were performed to obtain energy efficiency ratio models of four RIEC systems. Five climate zones in the Mediterranean region were considered for calculation of the SEER values. A detailed calculation method based on annual energy simulations was employed, and the results of the seasonal index were compared with those of a simplified calculation method based on the steady-state operation at four specific test points. The relative error between the SEER calculation methods was 4.6% for all the RIEC systems and climate zones. The highest SEER values of 5.8 and 5.5 were achieved for the Cairo and Madrid weather conditions, respectively. This study can serve as a reference for research on the seasonal performance and life-cycle analysis of RIEC based on an efficient counterflow heat exchanger with a wide range of airflow and cooling capacities.

Kinetic consistent MHD algorithm for incompressible conductive fluids

The first ideas concerning the principle of operation of non-mechanical magneto-hydrodynamic pumping systems were originally created at the end of the 1960s during studies of efficient heat exchange systems in nuclear energy sources using liquid metals as coolant. Currently, these ideas are of increasing interest in the creation of many critical technologies that require precise flow control in distributed energy transfer systems, in particular for a new generation of environmentally friendly efficient nuclear power sources.In this regard, computational fluid dynamic methods are an effective instrument for studying complex energy transfer processes, the possibilities of which have been significantly expanded with the creation of modern high-performance computing systems and modern computing technologies.The purpose of this article is to describe an advanced method of the mathematical modeling of magneto-hydrodynamic processes based on kinetic and kinetically consistent models. The novelty of this approach lies in more advanced physical models derived from the Boltzmann kinetic equation with a complex-valued statistical distribution function that includes electromagnetic interactions. The possibility of applying this description to incompressible viscous media (liquid metals) in complex magneto-hydrodynamic systems is shown.The system of equations of the proposed method, a numerical algorithm and the results of the mathematical modeling of a test system for energy transfer using a magneto-hydrodynamic pump and an exchange cavity are presented.

Thermal Response Measurement and Performance Evaluation of Borehole Heat Exchangers: A Case Study in Kazakhstan

The purpose of the present work was to determine the thermal performance of borehole heat exchangers, considering the influences of their geometric configurations and the thermophysical properties of the soil, grout and pipe wall material. A three-dimensional model was developed for the heat and mass transfer in soil (a porous medium) and grout, together with one-dimensional conductive heat transfer through the pipe walls and one-dimensional convective heat transfer of the heat transfer fluid circulating in the pipes. An algorithm was developed to solve the mathematical equations of the model. The COMSOL Multiphysics software was used to implement the algorithm and perform the numerical simulations. An apparatus was designed, installed and tested to implement the thermal response test (TRT) method. Two wells of depth 50 m were drilled in the Almaty region in Kazakhstan. Gravel and till/loam were mainly found, which are in accordance with the stratigraphic map of the local geological data. In each well, two borehole heat exchangers were installed, which were an integral part of the ground source heat pump. The TRT measurements were conducted using one borehole heat exchanger in one well and the data were obtained. The present TRT data were found to be in good agreement with those available in literature. The numerical results of the model agreed well with the present TRT data, with the root-mean-square-deviation within 0.184 °C. The TRT data, together with the predictions of the line-source analytical model, were utilized to determine the soil thermal conductivity (λg = 2.35 W/m K) and the thermal resistance of the borehole heat exchanger from the heat transfer fluid to the soil (Rb = 0.20 m K/W). The model was then used to predict the efficiencies of the borehole heat exchangers with various geometric configurations and dimensions. The simulation results show that the spiral borehole heat exchanger extracts the highest amount of heat, followed by the multi-tube, double U-type parallel, double U-type cross and single U-type. It is also found that the spiral configuration can save 34.6% drilling depth compared with the conventional single U-type one, suggesting that the spiral configuration is the best one in terms of the depth and the maximum heat extracted. The simulation results showed that (i) more heat was extracted with a higher thermal conductivity of grout material, in the range of 0.5–3.3 W/m K; (ii) the extracted heat remained unchanged for a thermal conductivity of pipe material higher than 2.0 W/m K (experiments in the range of 0.24–0.42 W/m K); (iii) the extracted heat remained unchanged for a volumetric flow rate of water higher than 1.0 m3/h (experimental flow rate 0.6 m3/h); and (iv) the heat extracted by the borehole heat exchanger increased with an increase in the thermal conductivity of the soil (experiments in the range of 0.4–6.0 W/m K). The numerical tool developed, the TRT data and simulation results obtained from the present work are of great value for design and optimization of borehole heat exchangers as well as studying other important factors such as the heat transfer performance during charging/discharging, freezing factor and thermal interference.

Comparison of the Efficiency of Cross-Flow Plate Heat Exchangers Made of Varied Materials

This paper discusses a mathematical model for airflow through a cross-flow plate heat exchanger. The exhaust air is used to heat the supply air. Three kinds of plates are considered: made of aluminium, copper, and steel. The purpose of this research was to verify which material used to build the plate heat exchangers uses the exhaust air heat more efficiently. The method of the Trefftz function was used to determine approximate solutions to the analysed problem. The results obtained for 1.2 mm-thick plates and for external winter, summer, and spring–autumn temperatures are discussed. The results indicate that if the efficiency and price of the metals are considered, then steel is the best material for the plate heat exchanger. Thanks to the use of thin steel plates and the reduction in air exchange time to a few minutes, a cheap and efficient cross-flow heat exchanger can be obtained.

Global sensitivity analysis and optimal design of heat recovery ventilation for zero emission buildings

Energy-efficient building services are necessary to realise zero-emission buildings while maintaining adequate indoor environmental quality. As the share of ventilation heating needs grow in well-insulated and airtight buildings, heat recovery in mechanical ventilation systems is increasingly common. Ventilation heat recovery is one of the most efficient and viable means to reduce ventilation heat losses and save energy. Highly efficient heat exchangers are being developed or applied to maximise the energy-saving potential of heat recovery ventilation. Nevertheless, the effects of practical operating conditions and the constraints of heat recovery – such as variations in ventilation rates, frost protection, and the prevention of an overheated air supply over a long-term period, which may significantly influence realistic recovery rates – have been less considered in efforts to maximise the energy savings. It is unclear which design parameters for heat recovery devices have the greatest effect on the annual energy savings from ventilation.This study proposes annual efficiency and annual net energy saving models for heat recovery ventilation that consider ventilation rate variations, the longitudinal heat conduction effect and operating controls. We use a global sensitivity analysis to quantify the contributions of various design input parameters to the variation in annual recovery efficiency and annual net energy savings. We identify the most influential parameters and their significant interaction effects for the annual energy performance of heat recovery ventilation. More attention should be paid to these most influential parameters during the design process. Furthermore, the optimal designs for rotary heat exchangers (as identified by a pattern-search optimisation algorithm) can improve annual net energy savings in demand-controlled ventilation by 33–48%, depending on the building areas. In combination with the reference year analysis presented in this study, heat recovery and demand-controlled ventilation can help to meet the need for highly efficient ventilation systems and zero-emission buildings.

Efficiency and cost optimization of offset Plate-Fin heat exchanger

Plate-fin heat exchangers, are also called compact heat exchangers. Due to their small size, compared to other heat exchangers they are user friendly which allow greater flexibility in plant architecture. The heat exchanger is made by stacking up fin surfaces which are separated by a sheet. With the increase in the area of the offset fin structure, the heat transfer also increases. But on the other end total annual cost and entropy generation is also affected by geometric variables. In the present work, seven geometric variables of the heat exchanger are investigated for the optimization of effectiveness, total annual cost and entropy generation minimization. Genetic algorithm is implemented for the optimization. Finally, parametric analysis is also performed to investigate the effect of design variables on the optimized results.

A novel approach for suppressing flow maldistribution in mini-channel heat exchangers

Mini-channel heat exchangers are extensively used to achieve heat dissipation with high heat flux, while the problem of flow maldistribution seriously impairs the thermal efficiency and operational reliability of heat exchangers. In this paper, the flow distribution characteristics of mini-channel heat exchangers with the conventional manifold model are numerically studied, and the influence of flow maldistribution on heat transfer is analyzed. To suppress the problem of flow maldistribution, an innovative approach for improving fluid uniformity by the built-in spiral baffle in the inlet manifold is proposed, and the effects of spiral baffles with different pitches and radii on flow distribution are discussed. Besides, the heat transfer characteristics and pressure drop under different manifold models are also compared. Finally, the variable-pitch spiral baffle is introduced to realize the stepped regulation of flow distribution. The results show that the conventional manifold model has serious flow maldistribution and causes the deterioration of heat transfer in front-end channels, especially at high Reynolds numbers. The improved manifold model greatly improves the uniformity of the fluid and temperature distribution inside mini-channel heat exchangers on the premise of sacrificing a small amount of pressure drop. The optimal pitch and radius of the spiral baffle are 20 mm and 9 mm respectively, and the flow maldistribution in almost all channels is controlled within the range of ±15% at this case. In addition, the variable-pitch spiral baffle further improves the flow uniformity of the mini-channel heat exchanger, and shows better overall performance than the equal-pitch spiral baffle.