Channel Heat Exchanger Research Articles

Numerical study of the effects of pressure gradient on the heat transfer in the KMS heat exchanger channel

The work is devoted to studying the influence of the pressure gradient on the heat transfer intensity in the channel of the KMS air heater. The aim of the work is to develop methods for improving the heat engineering characteristics of a heat exchanger using a pressure gradient. Numerical simulation is implemented based on the RANS approach using the Code Saturne software package using the k-w-sst turbulence model. The simulation conditions were set in accordance with the manufacturer’s instructions for the KMS-2 type air heat exchanger. In the work, the heat transfer coefficient was studied in the presence of a longitudinal pressure gradient in the heat exchanger channel from the gas path. As a result of the simulation, data were obtained on the effect of the pressure gradient on the heat transfer intensity for different working fluids in the channel of an advanced heat exchanger.

Research on flow channel layout of wing fins of PCHE heat exchanger based on SCO2

Supercritical carbon dioxide Brayton cycle is a new generation of thermal energy cycle system. As an efficient heat exchange channel of printed circuit board heat exchanger, compact microchannel has important research content.In this paper, using supercritical carbon dioxide as the working medium, a non continuous staggered heat transfer fin with sinusoidal arrangement of wing type is proposed. The effects of the structural characteristics of the fin and channel on the heat transfer efficiency under different channels are analyzed and compared with the traditional broken line fin.The comparative simulation results show that under the same working conditions, the flow and heat transfer performance of the wing sinusoidal staggered heat exchange channel is better, and the pressure drop increases by about 31.14%.

CFD-Based Simulation and Analysis of Hydrothermal Aspects in Solar Channel Heat Exchangers with Various Designed Vortex Generators

The hydrothermal behavior of air inside a solar channel heat exchanger equipped with various shaped ribs is analyzed numerically. The bottom wall of the exchanger is kept adiabatic, while a constant value of the temperature is set at the upper wall. The duct is equipped with a flat rectangular fin on the upper wall and an upstream V-shaped baffle on the lower wall. Furthermore, five hot wall-attached rib shapes are considered: trapezoidal, square, triangular pointing upstream (type I), triangular pointing downstream (type II), and equilateral-triangular (type III) cross sections. Effects of the flow rates are also inspected for various Reynolds numbers in the turbulent regime (1.2 × 104–3.2 × 104). The highest performance (η) value is given for the II-triangular rib case in all Re values, while the square-shaped ribs show a significant decrease in the η along the achieved Re range. The η value at Remax is 2.567 for the II-triangular roughness case. Compared with the other simulated cases, this performance is decreased by about 3.768% in the case of I-triangular ribs, 15.249% in the case of III-triangular ribs, 20.802% in the case of trapezoidal ribs, while 27.541% in the case of square ribs, at the same Remax. Also, a comparison is made with air-heat exchangers that have non-rough walls and contain cross-shaped VGs presented previously, in order to highlight the effectiveness of the rough surface presence in the baffled and finned channels. The obtained results indicated that the triangular-shaped rib (type II) has the most significant hydrothermal behavior than the other cases. This indicates the necessity of roughness heat transfer surfaces for finned and baffled channels to improve significantly the performance of the air-heat exchangers they contain.

CFD-Based Simulation and Analysis of Hydrothermal Aspects in Solar Channel Heat Exchangers with Various Designed Vortex Generators

CFD-Based Simulation and Analysis of Hydrothermal Aspects in Solar Channel Heat Exchangers with Various Designed Vortex Generators

Experimental investigation on heat transfer characteristics of two-phase flow boiling in offset strip fin channels of plate-fin heat exchangers

To optimize the plate-fin heat exchangers in the cryogenic processing system, the heat transfer characteristics of two-phase flow boiling for high-efficiency and pressure-resistant offset strip fins in plate-fin heat exchangers were investigated experimentally. The experimental conditions cover heat fluxes of 0.7–5.5 kW/m2, mass fluxes of 64–128 kg/(m2·s) and vapor qualities of 0.1–1.0. The research results show that, as the vapor quality increases, the heat transfer coefficient initially increases and then decreases, representing a maximum at vapor quality of 0.75–0.85; the criticalconditions of the bubbly-annular transformation and the annular-mist transformation are corresponding to the void fractions of 0.92 and 0.985, respectively. A new correlation of two-phase heat transfer coefficient for flow boiling in offset strip fin channels was developed based on the experimental data in the present study and in the literature, and the prediction deviation is within ±25%.

Enhancement of the turbulent convective heat transfer in channels through the baffling technique and oil/multiwalled carbon nanotube nanofluids

An attempt is made to improve the overall performances of channel heat exchangers. The techniques of baffles and nanofluids are combined to enhance the dynamic and thermal behaviors within the channel exchanger. Baffles under various attack angles are used as vortex generators. In addition, oil/multiwalled carbon nanotubes (MWCNT) is used as a working fluid. Both inclinations in the upstream and downstream directions were considered, referenced as Case A (UIB) and Case B (BIB), respectively. While the channel equipped with vertical baffles is referenced as Case C. The proposed models with combined techniques allowed a considerable enhancement in the overall efficiency. The comparison between the three cases revealed that the most significant value of thermal enhancement factor (TEF) of 5.634 was reached with vertical baffles (Case C) at the highest value of Reynolds number. When using inclined baffles, the 75° upstream attack angle (Case A) allowed the highest TEF of 4.814, compared with Case B.

Research on head cooling of high-speed aircraft by liquid nitrogen

AbstractIn the development process of high-speed aircraft, the head of the aircraft is subject to high temperatures and high speed flows, supporting the maximum heat flow and thus requiring a reliable cooling system. A new type of head cooling system is proposed herein. An internal flow channel model of the heat transfer in a ball head made from high-temperature alloy steel is constructed, then an experimental platform is built to carry out relevant experiments on the performance of this cooling system. Firstly, the influence of different experimental conditions on the cooling efficiency of the ball head is studied. For given liquid-nitrogen supply pressure, a higher heating heat flux density on the outer surface of the ball head corresponds to higher cooling efficiency. Then, the vaporisation effect under different experimental conditions is evaluated using temperature sensors at the inlet and outlet of the ball head heat exchange channel in combination with images of the visualised glass tube. It is found that liquid nitrogen can vaporise completely when flowing through the heat exchange channel. The characteristics of the heating effect and liquid nitrogen injection for the ball head were evaluated using an infrared camera. Finally, under different experimental conditions of liquid-nitrogen supply pressure, it is found that liquid nitrogen can vaporise completely in each case, and the total temperature of the vaporised nitrogen is about 300K. It can thus be collected as a secondary gas source.

Comparative analysis of the results of experimental and numerical studies of thermal and hydraulic characteristics of micro-hilly and corrugated plate surfaces

The experimental analysis of heat transfer in plate heat exchange channels, presented in this paper, was carried out on the installation of visualizing the temperature distribution on the surface of the electro-heated air flow cooled heat transfer envelope using a thermal camera. Experimental studies were carried out in a wide range of air flow. In the comparative analysis, the average deviation of the experimental values from the numerical simulation for the Nusselt criterion was about 15%. The worst systematic discrepancy between the Euler numbers (excess of the calculated losses over the experiment) is observed for the hydraulic characteristics of the hilly surface, but it does not exceed +22% in the transition region, decreasing to +14% in the region of developed turbulence. Within these errors, it is possible to use numerical 3d calculations for the development of databases of criteria dependences of corrugated and hilly heat exchange surfaces. To calculate the full-size heat exchangers obtained by calculation criterion dependences for different geometric characteristics of the models of corrugated and hilly surfaces are presented.

Numerical and experimental investigation of high concentration aqueous alumina nanofluids in a two and three channel heat exchanger

Abstract Nanofluids are often able to provide a method to increase the possible heat transfer of a system, with relatively few detrimental factors created by its inclusion. The use of nanofluids and their optimal concentrations has become an area of great interest as of late, with different nanofluid concentrations being key to a systems success or hindrance. The aim of this work is to examine the impact of nanofluid concentration on the possible heat transfer for different heat sink types containing porous media. In this work, both a numerical and experimental investigation into the impact of an aluminum oxide nanofluid on a fluid flow-based system comprised of porous open-cell aluminum foam to examine their impact on the thermal performance of the system was conducted. The porous media implemented in the experimental work was made of a 6061-T6 aluminum with a porosity of 0.91 and a permeability of 2.3869 × 10−7 m2. The experimental work was carried out for a large range of applied heat flux values varying from 3.8328 W cm-2 to 10.3737 W cm−2. The nanofluid examined was also subjected to varying flow rates. The nanofluid concentration was varied from 1% vol and 2% vol Al2O3–water mixture. The nanofluid was suspended in distilled water and mixed using a magnetic mixer. The performance of the work was examined through the Nusselt number to determine the possible thermal enhancement presented by the nanofluid. The use of high concentration (1% vol) nanofluid paired with the use of two different channel designs, both containing porous media resulted in an average thermal enhancement 15% and a maximum enhancement of 24.5% when compared to that of the 2% vol alumina nanofluid.

АКТУАЛЬНОСТЬ МОДЕЛЕЙ ЗАГРЯЗНЕНИЯ ДЛЯ ДИАГНОСТИКИ СОСТОЯНИЯ ПЛАСТИНЧАТЫХ ТЕПЛООБМЕННИКОВ

The article analyzes various theoretical approaches to describing the processes of formation of scale layers on the working surfaces of heat exchangers. The main tasks include analyzing existing models of contamination of heat exchange channels, determining the main mechanism of salt deposition on the heating surface of plate heat exchangers of heat supply systems, determining the main factors that determine the intensity of salt deposition on the working plate in accordance with the dynamics of heat and hydraulic processes in heat exchange channels formed by corrugated plates, as well as forming trends for further research. The article presents the main results of research devoted to the study of contamination processes on heating surfaces. Inaccuracies in the proposed approaches to describing the nature of the formation of salt deposition layers are identified. By generalizing existing approaches to the mathematical description of pollution processes, the main assumptions are revealed when describing the processes of salt deposition on working plates. A hypothesis is proposed about the influence of the location of channels relative to the inlet pipe on the uniformity of the flow distribution between parallel channels in the device. There is a fairly large gap between the existing computational methods for modeling pollution processes and the actual distribution of scale layers during the operation of heat exchange equipment of heat supply systems

Numerical simulation of heat transfer during the liquid cooling of complex surfaces at unsteady heat release

The presented mathematical model allows simulating the heat transfer process during the liquid cooling of complex surfaces under conditions of unsteady heat release. The numerical simulation can be provided for the case of film cooling as well as for channel heat exchangers. The presented simulation approach is based on the Finite Volume Method (FVM) which makes it possible to perform the calculations for heat exchangers with complex geometry. Usage of the Volume of Fluid (VOF) method allows simulating the heater cooling by the liquid film with a free surface. The presented mathematical model was tested on a set of specific problems. The simulation results are in satisfactory agreement with the known solutions.

Experimental study of R744 heat pump system for electric vehicle application

In this paper, two types of refrigerant, R134a and R744, are studied with different heat pump (HP) structures. Gas cooler, evaporator and indoor gas cooler (IGC) are all micro channel heat exchangers. One four-way valve and a combination of accumulator with internal heat exchanger (ACCU/IHX) are applied in R744 HP system, which can make the heat pump system structure simpler. In addition, one bidirectional throttle valve is applied before the evaporator to obtain the cooling and heating functions for R744 HP system. And the indoor gas cooler and evaporator are in series structure not only for the heating mode but also for the cooling mode in order to increase the heat transfer surface in the R744 system. The system charging test is carried out first to determine the optimal charge amount for the whole system, since it is essential for the stability of the system. Then the R744 and R134a heat pump system cooling and heating performance are studied by the experiment at different ambient working conditions. Test results show that the R744 system proposed in this paper can achieve similar cooling capacity and COP to that of the R134a heat pump. For R744 system, under 40℃ ambient working condition with external air circulation, the cooling capacity can reach 7524 W with COP of 1.36. And the R744 heat pump system can provide higher heating capacity and higher COP than the R134a heat pump system. Especially in the very cold weather, such as the −20℃ and −25℃, the R744 heat pump still can provide enough heating capacity with higher COP. Compared to the traditional R134a heat pump system, the heating capacity of R744 system increases by 83%, reaching 7378 W (R134a, 3994 W) at 6000 revolutions per minute (RPM) under −10℃ ambient temperature conditions. Under −20℃ ambient temperature with external air circulation, the maximum heating capacity is 7502 W with COP of 2.16.

Numerical Analysis with Experimental Validation of Single-Phase Fluid Flow in a Dimple Pattern Heat Exchanger Channel

A plate heat exchanger with a dimple pattern heat plate has a large number of dimples. The shape of dimples defines the characteristics of the plate heat exchanger. Although such heat exchangers have become increasingly popular due to their beneficial characteristics, knowledge of the flow characteristics in such kind of channel is poor. A good knowledge of the flow conditions inside of such channel is crucial for the successful and efficient development of new products. In this paper single-phase water flow in dimple pattern plate heat exchanger was investigated with application of computational fluid dynamics and laboratory experiments. Numerical analysis was performed with two turbulence models, Realizable - with enhanced wall treatment function and - SST. The first predicts a slightly smaller pressure drop and the second slightly larger compared to the results of laboratory measurements. Our research found that despite the relatively low velocity of the fluid, turbulent flow occurs in the channel due to its shape. We also found that there are two different flow regimes in the micro plate heat exchanger channel. The first regime is the regime that dominates the heat transfer, and the second is the regime where a recirculation zone appears behind the brazing point, which reduces the surface for heat transfer. The size of the second regime does not change significantly with the velocity of the fluid in the volume considered.

Combination of baffling technique and high-thermal conductivity fluids to enhance the overall performances of solar channels

In the present study, two-phase flow and forced-convection heat transfer of hydrogen gas (H2) in a solar finned and baffled channel heat exchanger (SFBCHE) is studied numerically. The effect of different obstacles in the channel is addressed. A H2 heat transfer fluid (HTF) having a high thermal conductivity with the baffling technique is implemented to enhance the overall performance of a solar channel. In the initial step, the results from the proposed numerical model were compared with the experimental data of a smooth channel, and then against data with a baffled channel. After checking the validity of our model, the same numerical approach was used for studying thermal-fluid characteristics of the channel with the new fluid. A hydrothermal analysis is presented for a range of Reynolds number (Re) from 5000 to 25,000. At the lowest Re = 5000, the thermal enhancement factor (TEF) is about 1.25. This value increases to 2.16, or 73.46%, when Re = 10,000. This increase in the TEF values continues as Re increases. The largest Re = 25,000 gives the highest TEF value, as it is about 4.18, which is 2.75 times greater than that given for the case of using the conventional gaseous fluid (air). Therefore, our proposed structure for the SFBCHE with high H2 HTF flow velocity leads to improve the values of dynamic pressure (Pd) and heat transfer (Nu), while reducing the skin friction (f) values, which increases the overall TEF of the channel. In addition, all performance values are greater than unity (or 1.00). This reflects the importance of the H2 HTF baffling and finning technique in improving the hydrodynamic thermal-energy performance of solar heat exchangers. The suggested model of SFBCHE filled with an H2 HTF having a high thermal conductivity allows a considerable enhancement in the overall thermal performances which can be employed in various thermal types of equipment, such as solar energy receivers, automotive radiators, and cooling in chemical industries.

Study the hydro and thermal performance of microchannel heat exchanger with hybrid suspension of nanoparticles and MEPCM particles

This paper aims to study the flow and heat transfer of hybrid suspension in counter flow micro channel heat exchanger (CFMCHE). In order to enhance the thermal properties of micro encapsulated phase change material (MEPCM) suspension. The hybrid suspension studied in this paper consists of nanoparticles and MEPCM particles, these particles are suspended in water as abase fluid, two types of hybrid suspension are used (Cu + MEPCM suspension) and (Al2O3+ MEPCM suspension) by n-octadecane as a PCM for both of them. The hydrodynamic and thermal characteristics of thes suspensions flow in micro channels of CFMCHE is numerically investigated. From obtained results, using of hybrid suspensions as a cooling fluid lead to modify thermal performance of a CFMCHE by increase its effectiveness but it also lead to high increasing in pressure drop. The results showed considerable enhancement in cooling effectiveness of (Al2O3 + MEPCM suspension) above of the pure PCM suspension, Nanofluids, and water. While (Cu + MEPCM suspension) showed enhancement above of the pure PCM suspension and water, but was not best effectiveness than Nanofluids. Extra increase in pressure drop in both types of hybrid suspension above are leads to reduce the overall performance compared with pure PCM suspension. Therefore its use depends on the application at which this heat exchanger is employed

Investigation of Refrigerant R134a Two Phase Flow Heat Transfer in Vertical Heat Exchanger Channel

Two phase flow boiling heat transfer and pressure drop in the vertical evaporator tube section of refrigeration system have been experimentally investigated using refrigerant R134a as a working fluid. The objective of the present work is to investigate experimentally the effect of heat flux, mass flux, vapor quality and saturation temperature on refrigerant flow boiling heat transfer characteristics in the evaporator of refrigeration system. These investigated parameters have significant impacts to enhance the thermal performance of the evaporator. The experimental investigations were conducted in smooth copper tube with inner diameter 5.8 mm and 600 mm length under different test conditions. The test conditions considered in this study were, for heat flux of 7.718-32.78 kW/m2, mass flux of 97.3-148.7 kg/m2.s , saturation temperature of -20.58 to -15.68 ?C and vapor quality of 0.3-1. It can be concluded from the results that, the average heat transfer coefficient at relatively greater mass flux 148.7 kg/m2.s was higher in range of 21% compared to other mass fluxes at constant test conditions. The relatively higher value of heat transfer coefficient was observed at heat flux 32.78 kW/m2 with average increase of 19% compared to the relatively lower value 23.38 kW/m2. The enhancement in local heat transfer coefficient at saturation temperature -15.68?C was higher by about 12% than that for the relatively lower temperature -20.58?C. The effect of increase in mass flux and heat flux on pressure drop in the evaporator tube was about 9% and 7.5% respectively.

Deposition characteristics of particles in inclined heat exchange channel with surface ribs

Ash particles in flue gas flow would deposit on the heat recovery exchanger in the waste heat recovery system (WHRS), which may lead to the reduction of heat transfer efficiency and service life. Understanding of ash particle deposition in the heat exchange channel is crucial for the safe operation of WHRS. In this study, deposition characteristics of ash particles in an inclined heat exchange channel with surface ribs were investigated by using Reynolds stress model combined with discrete phase model. The turbulent fluctuation correction and particle rebound model were developed by using UDF program to improve the prediction accuracy. After grid sensitivity analysis and numerical verification, the flue gas thermal-fluid fields, the 3D particle deposition patterns and deposition efficiency on each ribbed channel surface were obtained and analyzed. Parameter study on the deposition characteristics was conducted by considering flue gas flow velocity, particle size and inclination angle of the ribbed channel. The results showed that significant secondary flow eddies can be observed near the rib windward surfaces and high-value turbulent intensity area appears near the rib crest. The windward surfaces and the crests of the ribs are the main deposition locations in the inclined heat exchange channel. The deposition efficiency in the ribbed channel is obviously changed by the particle diameter and flue gas flow velocity. The inclination angle has little influence on deposition efficiency for small particles (dp<5μm) but great effect on deposition efficiency for large particles (dp>10μm).

Enhancement of the Hydrodynamic Characteristics in Shell-and-Tube Heat Exchangers by Using W-Baffle Vortex Generators

Improving the hydrodynamic characteristics of STHECs (Shell-and-Tube Heat Exchanger Channels) by using BVGs (Baffle-type Vortex Generators) is among the common passive methods due to their proved efficiency. In this computational investigation, the same method is used to enhance the hydrodynamic behavior of STHECs, by inserting W-shaped Baffle-type Vortex Generators. The numerical model represented by the computational FVM (Finite Volume Method) is used to simulate and analyzed the considered physical model. The fluid used is air, its thermal physical properties are constant, turbulent, incompressible, and its temperature is 300 K at the inlet section of the STHEC. The flow velocity ( Uin ) and atmospheric pressure ( Patm ) are considered as boundary conditions at the entrance (x = 0) and exit (x = L) of the channel, respectively. The results showed that the friction coefficients were related to the pressure, velocity, and Reynolds number values. High values of Re yielded an acceleration of the fluid, resulting thus in increased pressure on the solid walls and augmented friction values.

MODELING TRANSITIONAL CONDITIONS TO CHARACTERISTIC MODES OF THE COURSE OF TWO-PHASE CRYOGENIC MEDIA IN PIPES AND RING CHANNELS

Adiabatic and diabatic two-phase flows are widespread in cryogenic technology. The organization of the forced circulation of boiling liquid in channels of various orientations is one of the effective ways of cooling numerous heat exchangers. However, to create a rational design of heatexchange channels, it is necessary to know the conditions corresponding to the transition to undesirable flow regimes, in the presence of which the heat transfer intensity along the perimeter of the horizontal channel is significantly impaired. A comprehensive study of hydrodynamics and heat transfer during forced movement of a two-phase flow of helium (as well as nitrogen) in pipes and annular channels with visual observation of flow regimes was conducted at a special experimental bench at USPTU. Conventionally, it is possible to distinguish several flow patterns of a two-phase flow in the horizontal channel that are characteristic of hydraulic resistance and heat transfer. This is a stratified flow, including stratified and wave flows, slug and wave flows with jumpers, as well as flow regimes that are little dependent on the channel orientation - bubble and ring, including wave-ring and dispersed-ring flows. In the annular channel, the flow regimes are in qualitative agreement with the indicated flow regimes in the pipe. However, one of the determining parameters for identifying the conditions for stratification of a two-phase flow in an annular channel is its outer diameter. In the projectile and wave modes with jumpers, liquid jumpers support the flow of a liquid at the upper forming channel wall. In this case, the reduced resistance (as a function of the vapor content) is on average close to the calculated one for the homogeneous model. The heat transfer intensity near the upper generatrix of the wall (for small thickness and heat conductivity of the wall) depends on the ratio between the rate of evaporation of the near-wall liquid film and the hydrodynamic characteristics of the projectile and wave flow with jumpers. So, at small values of the heat flux density, the heat transfer intensity on the upper generatrix of the wall is close to the heat transfer intensity on the lower generatrix. When the values of the heat flux density correspond to the region of developed bubble boiling, as a rule, the upper generatrix of the wall undergoes a transition to film boiling (more precisely, convective heat transfer in the gas phase), and heat transfer compared to heat transfer to lower generatrix decreases by about an order of magnitude. The resulting liquid jumpers cause a significant temperature pulsation on the upper forming wall. When the conditions corresponding to the transition to the annular flow are reached, the heat exchange intensifies on the upper forming wall.

Effect of Dimensionless Number (KN and RE) on Performance Index of Concentric Circular Micro Channel

Overall performance of any type of heat channel is largely depends upon Knudsen number and Reynolds number [1]. In present work the outcome of dimensionless number on heat transfer properties of micro channel heat exchanger [2]are investigated. CFD analysis of counter flowmicro channel was performed for both slip and no slip flow using Fluent as CFD code. Based on results obtain, pressure drop increases with increment in Reynolds number and Knudsen number. The effectiveness reduces with increment in Reynolds number. The comparison of thermal and hydrodynamics performance between slip and no slip flow for heat exchanger is investigated. The effectiveness increases with decreasing values of Reynolds number for counter flow. For higher effectiveness of the micro channel heat exchanger, Reynolds number should be less.