Channel Heat Exchanger Research Articles

Vortex Structurization of Swirl Flows in Energy Systems

The physical factors and conditions leading to the formation of stable vortex structures in internal turbulent swirling currents occurring in complex channels of various energy and heat exchangers are considered. The possibility of applying the theory of screw flows and the equation for the rate of helicity change to determine the critical conditions for the occurrence of a deterministic vortex structure of turbulent flows and to predict the effects of the occurrence of large-scale vortex formations in various elements of energy systems is substantiated. The phenomenon of the swirl flow crisis is described. It is shown that the use of thermodynamic stability analysis to describe complex vortex flows makes it possible to expand the applicability of mathematical modeling methods to describe the processes of formation of vortex structures. The results of computational and theoretical modeling of vortex formation processes during the flow of a swirl flow in pipelines of variable cross-section, which are elements of pipeline systems of a nuclear power installation, are presented.

Investigation on Flow Maldistribution and Thermo-Hydraulic Performance of PCHEs with Spoiler Perforated Boards

In this study, the effects of the maldistribution coefficient on the thermo-hydraulic performance of discontinuous fin printed circuit heat exchanger (DF-PCHE) entrance head and channels are numerically investigated. To improve the flow uniformity at the entrance head, the flow in the exchanger with three types of spoiler perforated boards (SPBs) having 3 × 3, 4 × 4, and 5 × 5 holes and three kinds of hole diameters (Φd = 30, 25, and 20 mm), respectively, are compared to the flow in an exchanger with no SPB. The results show that a small maldistribution coefficient for the inlet velocity field is beneficial for the thermo-hydraulic performance of the DF-PCHE channels, and a maldistribution coefficient of 0.7 is an acceptable velocity distribution for the PCHE channel inlet. Using the 3 × 3 SPB with Φd = 30 mm, the maldistribution coefficient becomes 0.7, the fastest among all the SPB application cases at ΔL = 150 mm. Moreover, its heat transfer coefficient and pressure drop increase by 22.46% and decreases by 47.2% compared to those of the exchanger without SPB, respectively.

Dynamic simulation model for three-wheel air-cycle refrigeration systems in civil aircrafts

Dynamic simulation model of air-cycle refrigeration systems in civil aircrafts is essential for revealing the operation mechanism during flight process. In the present study, a dynamic model of three-wheel air-cycle refrigeration system was developed. A near-logarithm heat transfer temperature difference function is utilized to improve the stability of the heat transfer solution. A dynamic solution method based on continuity equation and theorem of momentum was presented as the framework of the dynamic mass transfer solution. A pressure-flow decoupling method is presented to solve the dynamic pressure in the high-pressure zone, resolving the absence of key pressure parameters. The model has been validated based on experimental data, with a pack discharge temperature deviation of 0.05K. Dynamic simulations have been conducted with actual flight data to obtain transient performance under airborne condition. In the takeoff and landing phase, the drastic change in the environment results in low discharge temperature, endangering the condenser with icing risk, which can be avoided by enabling the TCV. In the cruise phase, the discharge temperature slightly exceeds the upper limit for a couple of minutes. The optimization by slightly extending the length of heat exchanger channels succeeded in eliminating the excessive discharge temperature with minimal side effects.

Digital image analysis of gas-liquid flow in a cross-corrugated plate heat exchanger channel: A feature-based approach on various two-phase flow patterns

High-resolution photographs of the air-water flow in a transparent cross-corrugated channel are analyzed using digital image processing. Gas-liquid distribution is controlled via several inlet nozzles. Flow direction is varied between upward, downward, and horizontal flow.The image analysis algorithm is applied to a wide range of void fractions and flow patterns including bubbly, intermediate and film flow. Large bubble clusters are separated using edge detection. Local film and bubbly flow are identified with texture analysis. The method is suitable for locally inhomogeneous two-phase flow which is difficult to analyze visually.Objective and reproducible results are obtained by measuring features in the processed images: Mean and maximum bubble diameter, and the ratio of local film flow are determined. For non-uniform gas injection, expansion of two-phase flow across channel width is measured. Impact of flow direction and maldistribution on two-phase flow is demonstrated by calculating differences of the features. Uncertainties of image processing results are quantified. Bias is discussed based on visual analysis of the photographs in a previous study.Models for maximum bubble diameter and local film flow ratio, used to model flow pattern transitions in a previous study, are verified: For uniform two-phase distribution, models generally agree with measured values.

Comparative Thermohydraulic and Energy-Economic Characteristics of Heat Exchange in Round and Rectangular Channels Under Laminar and Turbulent Heat Carrier Flow Conditions

Comparative Thermohydraulic and Energy-Economic Characteristics of Heat Exchange in Round and Rectangular Channels Under Laminar and Turbulent Heat Carrier Flow Conditions

Numerical study of heat transfer, flow fields, turbulent length scales, and anisotropy in corrugated heat exchanger channels

In this study, we report on large eddy simulation (LES) of convectively dominated heat transfer in a corrugated heat exchanger channel using the computational fluid dynamics toolbox, OpenFOAM. A chevron pattern domain with 63 contact points is used to represent the conditions in a real plate heat exchanger (PHE). The unsteady nature of the flow is elucidated using visualization techniques based on volume rendering of temperature and iso surfaces of vorticity defined using the λ2-criterion and contours of wall shear stress and wall heat flux to illustrate the heat transfer process. Global surface averaged temperature and pressure drop are extracted from the LES on successively finer grids, approaching direct numerical simulation resolution, to increase the understanding of grid resolution requirements for LES in PHEs. Industry standard Reynolds-averaged Navier–Stokes simulations are compared to the LES results along selected profiles to demonstrate similarities and differences between the two techniques. The differences detected are further investigated using anisotropy invariant mapping, energy spectra, and turbulence length scale distributions. Significant differences between the model classes are detected and detailed. Moreover, the LES resolution requirements for the flow and the heat transfer processes are found to be different with the latter being more severe.

Experimental study of a pilot unit of a ground regenerator for greenhouses

The object of research: the process of heat exchange in a packing in the form of a dense layer of crushed stone as part of a thermal regenerator designed to utilize the low-grade heat of the air in a greenhouse.
 Investigated problem: increasing the efficiency of air heat accumulation in the greenhouse during the daytime by a regenerative heat exchanger with a granular packing. Conducting experimental tests of regenerator models with a packing in the form of a dense layer of granular material under natural conditions is necessary to improve the design and optimize technological parameters.
 The main scientific results: the analysis of temperature curves of air changes at the inlet and outlet of the regenerator installed in the mock-up greenhouse with a volume of 0.25 m3, and the packing material was carried out. During the heating period of the nozzle, the accumulated heat was Q=8•104 J, which can be used when the temperature in the greenhouse decreases. During the pause, the loss from the heat exchange channel to the environment is 48,000 J. The duration of the period was 358 min, while the average heat flux corresponded to 2.2 W. Heat losses during the pause period are significant due to its duration, however, the average heat transfer coefficient has a low value of k=2.3 W/m2K, so strengthening the insulation of the heat exchange channel is irrational.
 The area of practical use of the results of the study: greenhouses, allowing the possibility of installing additional heat exchange equipment.
 Innovative technological product: the innovativeness of the technical solution is determined by the possibility of using the heat of the air in the greenhouse and the implementation of contact heat exchange between the air flow and the packing particles. The design of the regenerator, the simplicity of its manufacture and operation, as well as the efficiency of air heat accumulation in the greenhouse during the daytime, allow to recommend it for use in greenhouse farms.
 The scope of the innovative technological product: regenerative heat exchangers with a dense layer of granular material for the utilization of low-grade air flows generated in food production enterprises, ventilation systems and greenhouses

Method for calculating the coefficient of hydraulic resistance of a coaxial heat exchanger with rough walls

A method for calculating the coefficient of hydraulic resistance of a coaxial heat exchanger with different roughness of opposite walls is proposed. According to the technique, the annular section of the heat exchanger channel is divided by a line of zero shear stress into two non-interacting annular layers. The balance equations in the layers and the channel together with the condition of matching the velocities of the layers on the zero shear stresses line form a closed system of equations. The solution of the system allows calculating the coefficient of hydraulic resistance of the channel as a whole, the position of the line of zero shear stresses, average velocities in each of the layers, the value of the second constant of the universal velocity profile. The method was verified in several ways, which showed the maximum deviation of the calculated values from the data of other authors and the results of our experiments, no more than 7%. In our experiments, we used three two-dimensional artificial roughness profiles: rectangular, trapezoidal, and triangular.

The influence of plate corrugation geometry on heat and mass transfer performance of plate heat exchangers for condensation of steam in the presence of air

Processes of steam condensation in the presence of air, as non-condensable gas, was studied in experiments with the models of plate heat exchanger (PHE) channels formed by plates with corrugations of different geometry. Validity of the developed mathematical model for the calculation of local process parameters is confirmed, and it is used to facilitate the analysis of experimental results. Sets of experiments were performed on five samples modelling PHE channel main corrugated field. The corrugations angle to plate vertical axis in three experimental samples was 30°, 45° and 60° at the same corrugations’ height of 5 mm. Another two samples had corrugations height 7.5 mm and 10 mm at the same corrugations angle 60°. Various effects of plate sizes and geometrical parameters of corrugations on the performance of PHE as a condenser for steam–air mixture and local process parameters profiles are discussed. Presented analysis shows that the specified process conditions on temperature and pressure drop in PHE can be satisfied by varying corrugations angle and channel spacing at plates of different lengths. To maintain the same temperature program with a decrease of plates corrugation angle from 60° to 30° require the plates with a length bigger from 2 to 2.6 times, depending on specific process conditions. These plate parameters are recommended for use as variables in solving the problem of PHE plate geometry optimisation with the use of the proposed mathematical model.

Effects of two-equation turbulence models on the convective instability in finned channel heat exchangers

Heat exchangers are multi-benefit thermal-devices, of wide use, as their structure varies with their scope of application. The development of channel configurations contained in these exchangers, and evaluation of their performance is the goal of many recent numerical and experimental achievements. Because of the high cost of such devices, many researchers had to follow CFD based computational methods instead of using experimental techniques. In this study, the hydrothermal structure of a finned channel heat exchanger is evaluated using the finite-volume based Ansys Fluent. Due to the complex structure of these channels, which contain overlapping transverse vortex generators, the flow develops into a turbulent structure that strongly influences the enhanced heat transfer. In this sense, various turbulence models are simulated to assess their numerical effect on the performance of the exchanger. Five different models are under experimentation which are (i) standard-, (ii) RNG- and (iii) realizable-case k-ε type models as well as (iv) standard- and (v) SST-case k-ω type models. As highlighted, the highest performance is reached with the SST-type k-ω, while the lowest one is that with the realizable-type k-ԑ.

Robust and general predictive models for condensation heat transfer inside conventional and mini/micro channel heat exchangers

To design, scale-up, and optimize the two-phase flow heat exchangers, accurate information about the heat transfer coefficient (HTC) is required. Here, an extensive database including 11,128 experimental data points from 80 sources, covering 37 condensing fluids over a wide range of operating conditions, were gathered for developing general and accurate models to forecast the HTC in mini/micro and conventional channel heat exchangers. The conventional intelligent approaches, namely, gaussian process regression (GPR), radial basis function (RBF), and hybrid radial basis function (HRBF) and also the least square fitting method (LSFM) were utilized for development of the new models. Firstly, the gathered data were used in evaluation of earlier two-phase HTC models, and these models showed average absolute relative error (AARE) values higher than 29%. Thus, more accurate models are necessary for these systems. Based on Pearson’s correlation analysis, eight dimensionless groups were selected as the most important input parameters. The GPR model showed the best results in estimation of Nu number for the test process with AARE of 4.50%. However, RBF and HRBF models estimated the HTC with AARE values 19.41% and 24.36% for testing data points, respectively. In addition, a general explicit correlation was developed by the least square fitting method (LSFM) for estimating the HTC, with acceptable results at an AARE of 23.40% for all analyzed data. The performance of the new general correlation, GPR and the previous models were assessed for different channel sizes, flow regimes, fluid types, and operating conditions. The new models and well-known earlier ones were examined with about 372 additional data samples from four new independent sources. The proposed models showed excellent results for prediction of HTC. Finally, a sensitivity analysis was studied based on the experimental data and the outcomes of the LSFM.

Computational analysis of a heat transfer characteristic of a wavy and corrugated channel

In a compact heat exchanger, optimal surface area utilization through the use of various channel and surface configurations contributes to creating a more efficient heat exchanger. In recent years, researchers have focused their attention on the usage of various types of channels and surfaces in heat exchangers, and a number of performance-based assessments have been published in this field. An in-depth thermal analysis of fluid flow and heat transfer within a wavy channel and corrugated channel heat exchanger is presented in this paper. Considering problem statement and the energy equation a two-dimensional computational model of the wavy and corrugated channel has been designed with the aim of simulation. The Navier-Stokes equation, which governs the flow, is solved with the help of the ANSYS fluent solver. Through the whole length of the channel, the effect of Reynolds number on various parameters such as surface Nusselt number, friction factor, wall shear, and pressure distribution has been explored to determine their relationships. According to the findings, the wavy channel has a better heat transfer capability than the smooth and corrugated channels when subjected to turbulent flow (with a high Reynolds number).

Digital Image Analysis of Gas-Liquid Flow In a Cross-Corrugated Plate Heat Exchanger Channel:A Feature-Based Approach to Quantify the Impact Of Two-Phase Distribution and Flow Direction on Flow Patterns

Digital Image Analysis of Gas-Liquid Flow In a Cross-Corrugated Plate Heat Exchanger Channel:A Feature-Based Approach to Quantify the Impact Of Two-Phase Distribution and Flow Direction on Flow Patterns

INTENSIFICATION AND STUDY OF HEAT TRANSFER AND FRICTION IN THE PLATE HEATING SURFACES OF THE KMS-2 AIR HEATER WITH A PRESSURE GRADIENT

A numerical study of heat transfer and friction in the heat exchanger channels in the presence of a variable pressure gradient is performed. The research was carried out in software complexes (Code_Saturne, Salome). The results of the validation of the research method are presented and they showed that the deviation of the numerical simulation results from the calculation data according to the known criterion equations is within the error of generalization of the experimental data by the criterion equations. According to the results of studies at Red=3000, Red=4177, Red=6000, it was found that the average value of the heat transfer coeffi cient of the channel of variable cross-section is up to 20 % higher than for the channel at dp/dx0. At the same time, the thermal-hydraulic effi ciency of the alternating channel (L=117 mm, l=58.5 mm, n=2) in the initial section at x =0...0.08 is lower than in the channel with dp/dx>0 by 26.7 %, and at x =0.08...1 it is higher by 5 ... 15 %, at dp/dx

Interface Tracking Investigation of the Sliding Bubbles Effects on Heat Transfer in the Laminar Regime

Helium gases are utilized to remove fission products from the molten salt fast reactor (MSFR) core during operation. Helium gases and other volatile fission products may be introduced into the intermediate heat exchanger channels. The effect of these gases on heat transfer is essential for the MSFR to operate properly, especially in laminar flow regimes. The computational fluid dynamics code PSI-BOIL was selected to examine this problem because of its interface tracking capability. A periodic square duct simulation created the flow regime, resulting in a sliding bubble regime. Following that, we examined the impact of heat transfer using an extended nonperiodic channel simulation with a succession of corner bubble arrays. Due to the combined effects of low thermal diffusivity and laminar flow characteristics, it is shown that the length of heat transfer augmentation may extend to at least five bubble diameters downstream of the bubble placement. Finally, we examined the impact of interphasic heat transfer between an inert gas and a liquid. The bulk of the heat transfer amplification effect was due to the motion of the bubbles rather than interphasic heat transfer.

Experimental efficiency evaluation of the of various geometry twisted bands for heat transfer intensification in the heat-exchange channels of a nuclear power unit equipment

The paper is devoted to experimental study of heat exchange and pressure drop of channel with various geometry twisted bands. The studies were carried out in the range of operating modes parameters of the nuclear power units’ heat exchange equipment. The three different designs of intensifiers are presented in the paper. The values of heat transfer coefficient and pressure drop are obtained. The dependences of the Nusselt number (Nu) on the Reynolds number (Re) were calculated. The comparative analysis of intensifiers is made. The efficiency factor was also calculated on experimental data. The most optimal geometry form of intensifier was selected.

Numerical Heat Transfer Investigation in a Solar Receiver Heat Exchanger Channel with Punched Elliptical-Winglet Vortex Generators

Numerical Heat Transfer Investigation in a Solar Receiver Heat Exchanger Channel with Punched Elliptical-Winglet Vortex Generators

Details on the Hydrothermal Characteristics within a Solar-Channel Heat-Exchanger Provided with Staggered T-Shaped Baffles

Details on the hydrothermal characteristics of turbulent flows in a solar channel heat exchanger (CHE) are highlighted. The device has transverse T-shaped vortex generators (VGs). Two staggered VGs (baffles) are inserted on the lower and upper walls of the CHE. The working fluid is Newtonian and incompressible, with constant physical properties. The ANSYS Fluent 17.0 is utilized in this survey. The second-order upwind and QUICK schemes were utilized to perform the discretization of pressure and convective terms, respectively. The SIMPLE algorithm was employed to achieve the speed-pressure coupling. The residual target 10−9 was selected as a convergence criterion. The effects of the T-VGs’ geometrical shape and Reynolds numbers were inspected. At the baffle level, the wall effect was augmented due to the reduction of the passage area of flows, which is estimated here to be 55%, resulting thus in a considerable resistance to the movement of fluid particles. The thermal distribution is highly dependent on the flow structures within the CHE. Since the fluid agitation yields an enhanced mixing, it allows thus an excellent heat transfer. The most considerable rates of thermal transfer were obtained with high Re, which resulted from the intensified mixing of fluid particles through the formation of recirculation cells and the interaction with the walls of the T-VGs and the CHE. The T-baffles with intense flow rates yielded negative turbulent speeds and intensify the fluid agitation, which improves the thermal exchange rates.

Numerical analysis of flow and heat characteristic around micro-ribbed tube in heat exchanger system

In order to improve the energy utilization rate, this paper analyzes the flow around the cylinder in the tube-row channel heat exchanger through numerical simulation. Effects of micro-rib structure types (vertical rib and ring rib), number of ribs (N = 0, 4, 6, 8), height of ribs (H = 2, 3, 4 mm), pitch diameter ratio (ST/D = 1.5, 2, 2.5), mass fractions (ω = 0–0.5%) and Reynolds numbers (Re = 514–1205) on the flow and heat transfer characteristics of tube-row channel heat exchangers are analyzed. Results showed that TiO2-H2O nanofluids can improve the comprehensive performance of heat exchanger, and when the mass fraction is 0.4%, the tube-row channel heat exchanger has the best performance. For heat exchangers with different types and structural parameters, the adjustment of the number of ribs, the height of ribs, the ratio of transverse pitch diameter and the use of nanofluids can increase Nusselt numbers by 53.63%, 57.25%, 52.38% and 12.93% at maximum, respectively.

3D Investigations of Heat Exchange and Hydrodynamics in Channels with Modified Double V-Shaped Dimples and Hemispherical Protrusions between Them

3D Investigations of Heat Exchange and Hydrodynamics in Channels with Modified Double V-Shaped Dimples and Hemispherical Protrusions between Them