Thermal Performance Of Heat Exchanger Research Articles

Thermal performance of a mini-channel heat exchanger (MCHE) working with CNT/GNP-based non-Newtonian nanofluids

In this work, the heat transfer of the non-Newtonian nanofluids inside a wavy microchannels heat exchanger (WMCHE) in cross-flow configuration has been experimentally and numerically studied. The Reynolds number (Re) does not have unique definition for non-Newtonian fluids. The non-Newtonian carboxyl methyl cellulose (CMC) aqueous solution containing 0.2 mass% CMC is used as base fluid (BF). The single-walled carbon nanotubes (CNT) and graphene nanoparticles (GNP) were added to the BF in 0.1 mass% as two other test fluids. Here, an equation based on the assumption of laminar flow is used in order to evaluate Re as a function of experimentally measured pressure drop. However, this assumption needs a similar form of friction factor relation to that of Newtonian fluids that is verified based on numerical simulation. In the presented work, finite element method (FEM) was utilized to perform the numerical modeling through Comsol Multiphysics software. Results show that as flowrate and relative waviness(2A/2L) increase, the convective heat transfer coefficient could be intensified. In terms of pressure drop, it was seen that with increasing the flowrate and relative waviness(2A/2L) of nanofluids, pressure drop was intensified. The results are compared to the experimental data and showed good agreement. The proposed performance index implies that GNP/BF nanofluid is the best one and both of two kinds of wavy configurations enhance the heat transfer efficiency, although the wavy two configurations are better.

Experimental investigation on thermal-hydraulic characteristics in a traverse rib tube fitted with regularly spaced helical tapes

A synergy enhancement approach by simultaneously combing a traverse rib tube (TRT) with regularly spaced helical tapes (RSHTs) was devised to increase thermal performance of heat exchangers aiming for saving more energy. Air flow heat transfer and pressure drop tests were respectively performed under constant heat flow and isothermal conditions by analyzing different parameters comprising Reynolds numbers, pitch length ratios and space ratios. In contrast to the plain tube, the gained results in the alone use of TRT show that the heat transfer rate and flow resistance are respectively augmented by 35.3%-49.4% and 1.48–1.70 times. By further fitting RSHTs into the TRT, the heat transmission performance and friction loss are increased by about 108.1%-177.0% and 5.97–15.31 times, respectively. The flow patterns are dominated by transverse streams and swirl flows, which render enhanced turbulent intensity and improved fluid mixing in the air flow passage, leading to thermal enhancement. The maximum value in performance evaluation criterion of about 1.26 is achieved by both the TRT and the TRT fitted with RSHTs. Moreover, the empirical formulas are developed from experimental values with deviations within ±5% and ±8% for Nusselt number and friction factor, respectively. Compared with published studies of swirl flow devices, the proposed enhanced apparatus of the TRT fitted with RSHTs in present study can be regarded as a proper candidate for potential heat transfer enhancement applications.

Numerical Investigation of the Effect of Nanoparticle Diameter and Sphericity on the Thermal Performance of Geothermal Heat Exchanger Using Nanofluid as Heat Transfer Fluid

The geothermal heat exchanger system is one of the most energy-efficient and environmentally friendly building service systems. In the present study, CuO/water nanofluid was used as the heat transfer fluid to enhance the energy efficiency of the geothermal heat exchangers. A three-dimensional numerical model was employed to investigate the effect of nanoparticle diameter and sphericity on the thermal performance of the geothermal heat exchanger, and it was well validated against the experimental results of nanofluids in the geothermal heat exchangers. The numerical results showed that nanoparticles with a diameter of 5 nm and 50 nm were not recommended for the nanofluids used in the geothermal heat exchangers due to the performance efficiency coefficient lower than 1, and the optimum diameter was 40 nm, which had the highest performance efficiency coefficient (1.004875). Moreover, the spherical particle-based nanofluid was characterized by the 8.55% higher energy efficiency, in comparison to rod-shaped particle-based nanofluid. Therefore, the application of nanofluid in the geothermal heat exchanger can enhance heat transfer, and the proposed optimum particle diameter and sphericity could contribute to higher energy efficiency.

Thermal performance of diffusion-bonded compact heat exchangers

This work presents an experimental and theoretical evaluation of the thermal performance of a square straight diffusion-bonded stainless steel compact heat exchanger. A one-dimensional steady-state thermal model was proposed to predict the thermal characteristics of the heat exchanger. To validate the model and to study the thermal behavior of the heat exchanger, an experimental test apparatus was developed. The heat exchanger was tested in several combinations of Reynolds ranging from 2600 to 7500, representing transition to turbulent regimes. The temperatures were varied from 70 °C to 80 °C for the water and from 25 °C to 42 °C for the air, at the inlet of the heat exchanger, respectively. A good agreement between the experimental data and the analytical model was obtained.

The Effect of Groundwater Flow on the Thermal Performance of a Novel Borehole Heat Exchanger for Ground Source Heat Pump Systems: Small Scale Experiments and Numerical Simulation

New small-scale experiments are carried out to study the effect of groundwater flow on the thermal performance of water ground heat exchangers for ground source heat pump systems. Four heat exchanger configurations are investigated; single U-tube with circular cross-section (SUC), single U-tube with an oval cross-section (SUO), single U-tube with circular cross-section and single spacer with circular cross-section (SUC + SSC) and single U-tube with an oval cross-section and single spacer with circular cross-section (SUO + SSC). The soil temperature distributions along the horizontal and vertical axis are measured and recorded simultaneously with measuring the electrical energy injected into the fluid, and the borehole wall temperature is measured as well; consequently, the borehole thermal resistance (Rb) is calculated. Moreover, two dimensional and steady-state CFD simulations are validated against the experimental measurements at the groundwater velocity of 1000 m/year with an average error of 3%. Under saturated conditions without groundwater flow effect; using a spacer with SUC decreases the Rb by 13% from 0.15 m·K/W to 0.13 m·K/W, also using a spacer with the SUO decreases the Rb by 9% from 0.11 m·K/W to 0.1 m·K/W. In addition, the oval cross-section with spacer SUO + SSC decreases the Rb by 33% compared with SUC. Under the effect of groundwater flow of 1000 m/year; Rb of the SUC, SUO, SUC + SSC and SUO + SSC cases decrease by 15.5%, 12.3%, 6.1% and 4%, respectively, compared with the saturated condition.

Thermal performance of a zig-zag channel formed by two wavy fins mounted with vortex generators

Wavy fins and vortex generators can separately enhance the thermal performance of heat exchangers. A novel combination of wavy fins and vortex generators is proposed in the present study to optimize the thermal performance of heat exchangers. The effects of the longitudinal vortices and attack angle of the vortex generators on the thermal performance of the novel combination are numerically studied. The longitudinal vortices are strengthened and weakened when the fluid flows over the crests and troughs of the bottom wall, respectively. The heat transfer on the pressure sides of both top and bottom wall surfaces is significantly larger than that on the corresponding suction sides. There is an optimal attack angle of 60°, which can provide the best heat transfer performance with a maximum 31.5% increase in the Nusselt number with only a 23.4% increase in the friction factor. The maximum thermal performance factor can be up to 1.23 with the optimal attack angle. Correlations for Nu and f are reported for both laminar and turbulent flows.

Thermal performance of the vertical ground heat exchanger with a novel elliptical single U-tube

Ground-Coupled Heat Pumps (GCHPs) are known as a promising technology for building climatization and domestic hot water production. The main concern to employ these systems in urban areas is their high drilling and initial cost. This cost could be rebated by lowering the borehole thermal resistance and by shortening the borehole depth. In the present paper, a novel and simple design of the Ground Heat Exchanger (GHE), namely the vertical GHE with an elliptical U-tube, is proposed to improve thermal performance of GCHP systems. To prove that; a series of 3D finite element simulations are performed to study the thermal comportment of elliptical U-tubes and to compare their performance with typical single U-tubes. A dimensionless shape factor γ is introduced to evaluate effect of the elliptical U-tube geometry on thermal performance of the GHE. Furthermore, influential parameters on thermal performance of elliptical U-tubes are investigated and the decrement percentage of the borehole thermal resistance, with reference to the typical U-tube, are compared for each parameter. The results show that elliptical U-tubes could enhance heat transfer to significant extent and could decrease the borehole thermal resistance by more than 17 %, compared with typical single U-tubes. It is shown that the higher value of the shape factor γ leads to the better heat transfer enhancement. It is concluded that elliptical U-tubes are able to decrease the borehole thermal resistance and to boost the coefficient of performance (COP) of GCHP systems.

The Effect of Shank-Space on the Thermal Performance of Shallow Vertical U-Tube Ground Heat Exchangers

One parameter that may affect the performance of a ground source heat pump is the shank-space, the center-to-center distance between the two branches of a vertical U-tube used in a ground heat exchanger. A 3D steady-state computational fluid dynamics (CFD) model of a U-tube ground heat exchanger was used to investigate the influence of varying shank-space on the thermal performance of two isolated vertical shallow U-tubes, one 20 m deep and the other 40 m deep, given that most existing research focuses on systems making use of deeper boreholes. The models adopt an innovative approach, whereby the U-junction at the bottom of the U-tube is eliminated, thus facilitating the computational process. The results obtained show that, although the temperature drop across the U-tube varies for different shank-spaces and is lowest and highest for the closest and the widest shank-spaces, respectively, this temperature drop is not linear with increases in shank-space, and the thermal performance improvement drastically diminishes with increasing shank-space. This indicates that, for shallow U-tubes, the temperature drop is more dependent on the length of the pipework.

Heat transfer analysis of a shell and tube heat exchanger operated with graphene nanofluids

Nanofluids have attracted huge attention because of their effective physical and thermal properties. One of many applications of nanofluids is the enhancement of the thermal performance of heat exchangers. In the current study, an experimental investigation has been conducted for studying the effects of graphene nanofluids on the convective heat transfer in a vertical shell and tube heat exchanger. Graphene flakes were prepared using graphite foam that is derived from sugar as a raw material. The prepared Graphene flakes have been characterized using scanning electron microscopy, X-ray diffraction, atomic force microscopy, and Raman spectroscopy. The graphene nanofluid has been used in the tube side of the heat exchanger to enhance its heat transfer performance. Different parameters such as nanofluids’ concentration, flow rate and inlet temperature were studied and their effects on heat transfer coefficient and thermal efficiencies are discussed. The results show that using of graphene/water nanofluids enhances the thermal performance of the vertical shell and tube heat exchanger. A maximum increase in the heat transfer coefficient of 29% was achieved using 0.2% graphene/water nanofluids. Furthermore, the mean thermal efficiency of the heat exchanger was enhanced by 13.7% by using graphene/water nanofluid.

Effect of wavy fin usage on thermal performance of heat exchanger used in combi boilers

In this study, performance of a heat exchanger used in combi boilers was investigated numerically for different fin geometries. Analyses were performed at the boiler operation conditions. A commercial CFD software package, FLUENT, was used for numerical simulations. The 3-D steady-state turbulent flow field analysis was carried out and k-? model was preferred as the turbulence model. In the analysis, it was assumed that the heat transfer phenomenon occurred both by conduction and convection. Flat fin geometry was taken as a reference for the investigation. Variation of the heat transfer and pressure drop values for the wavy fin were compared with the reference geometry. The wave angle and wave radius were taken as the parameters for the wavy fins. For different fin geometries: the outlet temperaTure of the combustion gases, the heat transfer to the water, and the pressure drop were calculated and the results were presented. Compared with flat fin, average decrease for the outlet temperature of hot gases was obtained as 4 K and average increase for the heat transfer to the water was calculated as 0.68 W. On the other hand, the average pressure drop in the heat exchanger with wavy fins was about 70% higher than the flat fin.

Comparative CFD analysis of helical shaped earth air pipe heat exchanger using water tube as a thermal reservoir using different pipe materials

Common configuration of EAPHE system usually consider single or U shaped configuration, which requires large installation area and are not suitable for installation under small home or office setup. This major drawback can be overcome by using helical shaped air pipe, which reduces installation area up to 60%. Performance of EAPHE deteriorates after long run due to thermal saturation in the soil near to pipe surface. In this paper problem of thermal saturation of soil is attempted by proposing water pipe of three different pipe materials. Water pipe act as thermal reservoir to absorb heat from surrounded soil and maintains constant heat transfer after 24 hours of long run. Thermal performance of Earth Air Pipe Heat Exchanger (EAPHE) has been investigated under four different type of model developed using helical air pipe and comparison were made in terms of temperature drop, soil temperature at 0.05m and 0.25 m from pipe, change in water temperature, COP and effectiveness of the system for the continuous 72hours of run.

Experimental Analysis of Effect of Soil Moisture on Thermal Performance of Earth Air Heat Exchanger

Earth Air Heat Exchanger (EAHX) systems are renewable energy systems based on the use of the high energy potential of the soil. Although it has a simple structure, there are multiple parameters that affect thermal performance. These parameters should be taken into consideration when installing the EAHX, which consists of a buried pipe system and a fan. In this study, the effect of humidification of the soil on which EAHX is placed by placing an irrigation line on thermal performance was investigated. In this study, two EAHX with the different material piping systems are discussed. In the study carried out at different airspeeds, inlet-outlet air temperatures, water temperature and air humidity were measured. It was determined that the decrease in the outlet temperatures of the systems using PVC and metal pipes was 10% compared to the dry condition. As a result of the study, it has been observed that moistening the soil increases EAHX efficiency. When the increase in cooling capacity and the additional cost caused by the irrigation system are compared, it is seen that the installation of soil humidification system is applicable in terms of energy efficiency.

Numerical Investigation on the Thermal Performance of Double Pipe Heat Exchanger Using Different Shapes of Fins

In this study, a numerical investigation on the thermo-hydraulic performance of thedouble pipe heat exchanger into heat transfer by different shapes of fins on the outersurface for the inner tube as extended surfaces. The inner and outer diameters of theinner pipe were (16.05 mm), (19.05 mm) respectively, and (34.1 mm), (38.1 mm) for theouter tube. The length of the heat exchanger was (1000 mm). Hot and cold water wereused as the working fluid, where the hot water flows inside of the inner one in counterflow with the cold water which flows in the annulus. The inlet temperature for the hotwater is (75 OC) while it is (30 OC) for the cold. The hot fluid flows at constant ratewhich is (0.1kg/s) while the cold is varied from (0.1 kg/s to 0.2 kg/s).The study wasperform using the known commercial CFD package (ANSYS – FLUNET 15) .Theresults shows that both (rectangular and triangular) fins enhances the heat transfercoefficient compare with the conventional plain tube .The rectangular fins presents anheat transfer enhancement ratio of (61% to 74%). Using of extended surfaces present agood result in saving energy by enhancing the performance of the double pipe heatexchangers used in petroleum industry.

Experimenting and Modeling Thermal Performance of Ground Heat Exchanger Under Freezing Soil Conditions

Many studies have investigated the thermal performance of ground heat exchangers (GHEs) under normal conditions with inlet temperatures above 0 °C, but the freezing soil condition has been absent. We conducted a three-month test to investigate the heat transfer of GHE with inlet water-glycol temperatures of −7~0 °C. An improved thermal resistance and capacity (RC) model was developed to investigate the heat transfer between vertical single U-tube GHEs and the frozen soil. After validating with experimental results and CFD simulations, the RC model was applied to analyze GHEs’ thermal performance under different freezing soil conditions. It shows that the frozen soil increases GHE’s heat transfer capacity by 30% and the freezing inlet temperature has limited impacts on temperature distribution around the exchanger (with < 4 m influence radius). The GHEs’ heat transfer rate remained at 75~80 W/m throughout the three-month test, which is surprisingly high to ensure the normal operation of the ground source heat pump (GSHP). These findings can be references for designing and operating GSHP systems in cold and severe cold climate zones, and the RC model can be applied to analyze future GHEs performance with phase change processes.

Extensive numerical analysis of the thermal performance of a corrugated tube with coiled wire

Utilizing passive methods is an efficient way in order to improve the thermal performance of heat exchangers. The novelty of the present paper is that two kinds of passive techniques for heat transfer enhancement, including a corrugated tube equipped with a coiled wire as turbulator, are combined in the present study. Heat transfer and pressure drop in a corrugated tube with a wire coil are analyzed numerically by a commercial CFD software. The range of the considered Reynolds number is 5000–20,000, which guarantees that the flow regime is fully turbulent. The effects of two different geometrical parameters and of two flow operating parameters, namely the Reynolds and Prandtl numbers, are numerically investigated. The conservative equations for momentum are closed using the k-e realizable model. Results indicate that the geometrical and fluid flow parameters have significant effects on the thermal performance of the system. When one increases the pitch ratio of the wire coil (P/d), the heat transfer enhancement ratio NuER decreases. The maximum decrease is obtained for P/d = 5 with − 7.78% at Re = 20,000 in comparison with the baseline case P/d = 2. The maximum increase in the performance evaluation criterion (PEC) is observed at Re = 5000: + 11.16% and + 17.4% for P/d = 3 and P/d = 5, respectively. The configuration with a roughness height equal to e = 3 mm exhibits the best thermal performance with a maximum enhancement in terms of NuER of + 3.3% at Re = 10,000 in comparison with the baseline case e = 0.5 mm. By considering the PEC number, increasing the parameter e has a negative effect on the thermal performance of the proposed system.

Thermal performance of shallow spiral-tube ground heat exchanger for ground-source cooling system

This study present an investigation of thermal performance of spiral-tube ground heat exchanger (GHE) buried in shallow depth of borehole. The spiral-tube GHE consisting of spiral pipe installed in the borehole provides a better performance in application of ground-source cooling system. Experimental study was carried-out by circulating water through the spiral-tube GHE which is buried in the ground of 3 m depth. Temperatures of inlet and outlet water in the GHE were measured and recorded periodically. The thermal performance of the GHE was calculated with different inlet water temperatures. Heat exchange rate for different inlet water temperature of the spiral-tube GHEs are 44 W/m (35 °C), 126 W/m (40 °C) and 110 W/m (45 °C). The results show that the utilization of shallow borehole of spiral-tube GHE is appropriated for application in ground-source cooling system especially for the hot climate like Indonesia.

Effect of Wet Soil on Thermal Performance of Air-Fluid Ground Heat Exchanger for Heating

The energy crises that emerged after the economic problems in the world increased the interest in alternative energy resources. The effects of global warming, which has a serious threat, will be reduced by the more efficient use of these energy resources. In this study, the thermal effects of wet soil were investigated experimentally using a ground source heat exchanger (GHE), which is an alternative energy resource, in an area on the Esentepe campus of Sakarya University. Researches on this subject are mostly directed to dry soil applications. In this research, the performance of soil in an artificial pool was examined in terms of heat transfer. By means of the artificial pool formed under the ground, it is aimed to increase in heat transfer between the soil and the process fluid. In the experiments which are conducted, air is used as the process fluid. The system has a significant advantage in certain temperature ranges due to the passive heating method, in other words, the process fluid can be circulated under the soil without using a compressor. The purpose of this method is to reduce the cost of heating in the winter season. The temperature difference at GHE inlet and outlet is approximately 9.07 °C in the experiments. The heat transfer rate has been increased by 46.28% compared to dry soil application for the same air velocity speed.

Augmentation of thermal performance of heat exchanger using elliptical and circular insert with vertical twisted tape

ABSTRACT Present work deals with the experimental and computational investigation of thermal properties and fluid flow characteristics on circular tube with elliptical inserts. The major axis is taken equal to hydraulic diameter i.e. 6.81cm. A new parameter, height ratio (γ = b/D) has been defined with three values – 0.45, 0.66, and 0.74. The test section is 1.5 m long which is provided with a constant heat flux of 1000 W/m2 along with air as working fluid at Reynolds number ranging from 3,000 to 21,000. The results show that the augmentation in heat transfer was six times for case γ = 0.74 and the maximum thermal performance factor (TPF) obtained is 2.43 for insert with γ = 0.45 at Re = 3000 over the smooth tube. Computational fluid dynamics (CFD) investigation is also carried out for elliptical inserts, and the results are in good agreement with the experimental results where the maximum deviations in Nusselt number, friction factor, and TPF are ±9.39, ±8.68, and ±8.58%, respectively. CFD investigation for tube having circular inserts with vertical twisted tapes (CVTT) and elliptical inserts with vertical twisted tapes (EVTT) is also presented where the augmentation in heat transfer ranges 3–8 times for CVTT placed in full length and TPF is ranging 1.19–2.63 times for CVTT placed in TL length over to smooth tube.

Experimental and numerical studies on the thermal performance of ground heat exchangers in a layered subsurface with groundwater

Thermal performance of the ground source heat pump (GSHP) system can be significantly affected by the complex geological substructures (such as ground stratification and groundwater advection). This study installed a seepage box inside the two-layered laboratory device to investigate the heat transfer processes of the unsaturated, saturated and infiltrated ground. Various operational and geological conditions were designed to study the temperature distributions at various locations and time experimentally and numerically. The groundwater effect on ground heat exchanger (GHE) thermal performance depends on the thermal properties, flow advection and the relationship between temperatures of the groundwater and ground. If the ground was partially saturated during the heat injection period, the cooling seepage will efficiently remove the heat of GHEs in upper-stream rather than those located in the bottom-stream. Meanwhile, the heat transfer can be enhanced if two legs of the U-tube vertical to the direction of groundwater seepage. The groundwater flow can redistribute the heat within the ground and showed a better recovery performance which advance an even temperature distribution by 3 h. The temperature and carried heat load of the cooler groundwater will increase during the heat injection experiment, and further contributed to various temperature distributions of ground at different locations and time.

Effects of Double V Cuts in Perforated Twisted Tape Insert: An Experimental Study

The thermal performance of heat exchanger is considerably enhanced by the use of inserts. Twisted tape inserts are known for producing large heat transfer enhancements at the expense of fairly large friction penalty. With the objective to explore the effect of twisted tape with perforations and V cuts, the present study investigates the thermo-hydraulic performance of solid twisted tape, perforated twisted tape, and perforated twisted tape with double V cuts for the Reynolds number range of 2000–25000. During the experiments, the twist ratio of tapes is varied in the range of 2–6, while the depth ratio and width ratio of V cuts for perforated twisted tape with double V cuts are varied in the range of 0.2–0.27. The maximum enhancements in Nusselt number and friction factor were found to be 3.5 and 9.63, respectively, corresponding to the perforated twisted tape with double V cuts having V cut depth ratio and width ratio of 0.27 and twist ratio of 2.