Heat Exchanger Header Research Articles

A method to calculate the two-phase distribution in a microchannel heat exchanger

Maldistribution seriously impacts the heat transfer performance of the microchannel heat exchanger (MCHX). Fully understanding the two-phase distribution in the heat exchanger is important for advancing academic research and engineering applications. This study introduces a novel method for quantifying two-phase distribution in a microchannel heat exchanger. An experimental setup was developed to measure the local vapor mass fraction in the heat exchanger header. Capacitance signals were measured under inlet vapor mass fractions from 0 to 1, and inlet flow rates of 10, 15, and 25 g s−1 corresponding to a mass flux of 17.47, 26.2, and 43.66 kg m−2s−1, respectively. The local vapor mass fraction in the header was estimated using the capacitance measurements. The mass flow rate in the header, the microchannel tube, and the vapor mass fraction in the tube were calculated using the proposed model. The calculation model was validated against literature data, and the results were analyzed. The analysis reveals the characteristics of vapor mass fraction and mass flow rate distribution in the MCHX and further elaborates on the effects of phase separation, entrainment ratio, and pressure drop balance on the distribution. The proposed method can evaluate distribution in the header and tubes of microchannel heat exchangers, and it is also applicable to other types of two-phase flow devices.

Quantification of two-phase flow in a microchannel heat exchanger header based on capacitance measurement

The two-phase distribution has always been one of the research focuses of microchannel heat exchangers, because the maldistribution could seriously affect the heat transfer performance. This paper developed a capacitive method to quantify the two-phase distribution of vapor-liquid flow in the aluminum header of a microchannel heat exchanger. A flexible capacitance sensor was used to quantify the two-phase distribution with R134a. The inlet mass fluxes vary from 17.47 kg m−2 s−1 to 52.42 kg m−2 s−1 and inlet vapor qualities range from 0.2 to 0.6. The normalized time-averaged capacitance signal was processed by a Gaussian function to obtain the mathematical expectation and the standard deviation. The local vapor quality in the header was solved for using a correlation derived from the pre-experiments. The distribution of local vapor quality showed that gases were concentrated upstream of the header, while liquids were concentrated in the middle and lower reaches. The local vapor quality reached a minimum and remained constant at 0.1 in the middle and lower reaches, unaffected by changes in inlet conditions. As the mass flux increased, both the local vapor quality maximum point and the minimum point in the header were shifted back toward the downstream. Additionally, the increase of mass flux caused the fluctuation of the standard deviation increased, which meant the stability of the flow decreased. The two-phase flow measurement technique based on flexible capacitance sensors proposed in this study realized the extension of the capacitive method from the experimental stage to the industrial application stage.

Condensation Heat Transfer and Pressure Drop of Acoustically Actuated Horizontal Two-Phase Flow

An acoustic actuation technique to enhance condensation inside horizontal tubes is developed and experimentally demonstrated. Heat exchanger headers are used as Helmholtz resonators, allowing low-frequency (∼10 Hz) acoustic waves to propagate and be reflected within the condenser, improving mixing saturated vapor and condensed liquid, thereby enhancing heat transfer. Heat transfer coefficients and pressure drops are measured to evaluate the heat transfer enhancement and pressure drop penalty as a function of vapor quality, mass flux, temperature difference, and acoustic wave amplitude and frequency. Experiments are conducted using a tube-in-tube heat exchanger with an inner diameter of 14.45 mm that partially condenses refrigerant R134a at 950 kPa across vapor qualities ranging from 5% to 80% as mass fluxes of 120 and 180 kg m−2 s−1 and temperature differences of 6 and 12 K. The actuation amplitude and frequency are varied from 0.5 to 6 mm and 2 – 20 Hz, respectively to characterize the acoustic response. Heat transfer enhancement mechanisms are hypothesized and evaluated based on the experimental results, concluding that changes in two-phase flow patterns and increased convective effects lead to significant enhancement at low qualities but negligible enhancement at high qualities when the flow is annular. Existing correlations and modifications to existing models are proposed to explain the observed enhancement, and a simple correlation based on a heat-and-momentum analogy is proposed, which predicts both baseline and enhanced heat transfer data within 2.7%.

Study of two-phase flow distribution in microchannel heat exchanger header - A numerical simulation

In the present paper, the two-phase flow distribution in the microchannel heat exchanger (MCHE) header is numerically studied, and the proper void fraction model, two-phase flow and turbulence models are obtained by comparing simulation results with the experimental results data. Numerical results show that the VOF model and SST k-ω turbulence model have a better performance in predicting two-phase flow distribution of vapor and liquid mass flow rates in each microchannel tubes under different refrigerant inlet qualities. Furthermore, the flow and phase distribution in a really-used longer MCHE header (length >400mm) with 55 microchannel tubes are numerically analyzed. Results indicate that the two-phase refrigerant in the distribution tube sprays from the distribution hole, and an annual liquid film forms on the inner wall of the header because of the frictional forces between wall and refrigerant.

Experimental results of a magnetically coupled piezoelectric actuator to relieve microchannel heat exchanger maldistribution

Refrigerant maldistribution is a common issue in microchannel heat exchangers. Refrigerant maldistribution can have significant negative effects on the heat transfer performance and increase the pressure drop, leading to increased superheat temperature at the outlet and increased compressor work, lowering system efficiency. A novel piezoelectric-driven magnetic actuator (PEDMA) was fabricated and inserted into the inlet header of the microchannel evaporator, with the goal of improving the non-uniformity of the flow distribution by regulating the flow in the header to the microchannels. Two prototype PEDMA designs were tested and showed significant impact on the refrigerant flow in the region of actuation. In the first design, the experimental data showed superheat reductions up to 9 °C when the individual channel temperatures were measured. The second design redistributed the refrigerant such that a 1% increase in capacity was calculated for the whole heat exchanger without significantly impacting the heating efficiency. The experimental results demonstrate the successful operation of the PEDMA devices and represent a new method of active flow distribution control which is based on a simple design with minimal additional energy consumption, and which can be inserted into microchannel heat exchanger headers to potentially relieve the refrigerant maldistribution.

The Influence of Header Design on Two-Phase Flow Distribution in Plate-Fin Heat Exchangers

AbstractFluid flow maldistribution causes deterioration of heat transfer as well as pressure drop penalty in heat exchangers. A test bench was set up to investigate the effect of different header designs on air-water flow distribution in plate-fin heat exchangers (PFHX). Two-phase flow distribution was examined for air Reynolds numbers (ReG) of 28293542 and inlet qualities (x) of 46.3–64.0%. Two-phase flow distribution was seen to be more uneven in the heat exchanger in comparison with single-phase flow, the water distribution being more uneven than that of the air. For a fixed inlet quality, as the air flowrate was increased, the distribution of two-phase flow became increasingly nonuniform, showing a pattern similar to single-phase flow. Furthermore, the air distribution became more even, while the water flow became more unevenly distributed as the inlet quality increased. To mitigate the maldistribution, perforated plates were incorporated in the heat exchanger header. The improved headers significantly aided in distributing the two-phase flow more evenly. At ReG = 2829 and x = 46.3%, the heat exchanger effectiveness was expressed in terms of the unevenness in quality, Sx. The effectiveness decreased as the unevenness of the flow distribution was exacerbated, emphasizing the significance of uniform phase and flow distribution as a key element of heat exchanger design.

Design study of microchannel heat exchanger headers using experimentally validated multiphase flow CFD simulation

Microchannel heat exchangers (MCHXs) are compact, light-weight, and are capable of providing high heat transfer performance. However, one of the major challenges with MCHXs is the maldistribution of two-phase refrigerant from the header into the microchannel tubes. This issue is especially prevalent at the inlet header in evaporator applications, which has two-phase refrigerant getting distributed into the MCHX tubes. Recent advances in multiphase flow CFD have made it possible to visualize and study the detailed multiphase flow behavior inside the MCHX header. The Hybrid Eulerian Multiphase (Hybrid EMP) model is used in this paper to perform a two-phase flow simulation of the refrigerant distribution. This model has the combined benefits of both the Conventional EMP model and the Volume of Fluid (VOF) model, where the former detects the dispersed phases while the latter captures the phase separation interfaces. The simulation domain is divided into 3 regimes. Two of the regimes use dispersed phase modeling, with one phase dispersed in another and vice versa. The third regime employs a large-scale interface approach to capture the separation between the two phases. The application of the Hybrid EMP approach is discussed in this paper, thoroughly explaining the developed CFD modeling including a sensitivity analysis of the sub-models and parameters. The model is experimentally validated using thermal infrared images. The flow behavior is discussed and the causes responsible for the maldistribution are identified. A design study is carried out for two types of vertical headers- a Tube-Insertion header and a Loop header, which has a loop added to the former type. The Loop header is found to show superior distribution performance. To understand the effects of the parameters critical to improving the distribution, a sensitivity analysis is performed for parameters like the tube-insertion depth, the inlet mass flow rate and the header loop dimensions. Overall, this paper is aimed at providing a comprehensive simulation approach of the header maldistribution problem in MCHXs.

Characterization and improvement of flow distribution in a vertical dual-compartment header of a microchannel heat exchanger

Abstract Poor distribution of two-phase flow in a microchannel heat exchanger header requires a solution to maximize its potential advantage of compactness while offering a high heat-transfer rate. While previous works have attempted to improve two-phase flow distribution, most have focused on horizontal headers, which commonly appear in indoor units. However, there are a few studies on vertical headers that function as an evaporator in heat pump mode that are typical in outdoor units. This study describes the development of a modified vertical header with a dual-compartment structure aimed at improving the flow distribution. Experiments and visualizations were conducted using R410A in a test section that represented the size and capacity of an actual heat exchanger. The inlet mass flux was 309–1235 kg·m−2s−1 (50–200 kg·h−1), vapor quality was 0.1–0.2, and the evaporating temperature was maintained at 15 °C. The results were compared to the conventional header previously evaluated by our research group. Results showed that the dual-compartment header was able to enhance the liquid flow distribution at low inlet mass flux from approximately 30% to 80% based on relative standard deviation (RSD), with a generally higher liquid Reynolds number when compared to the conventional header.

Leakage Analysis on Non-axisymmetric Gasket of the Shelltube Heat Exchanger Header

This paper focuses the flanged static sealing structure of shell-tube heat exchange header, builds an static sealing model based on FEM(Finite Element Method),studies the effect of bolt preload and working medium load on the gasket actual contact stress. The results indicate that: an approximately linear relationship exists among the gasket contact stress, bolt preload and working medium load. When the pressure is gradually increasing, the flat end-plate’s outward convex deformation results in the decreasing gasket stress in the center and even leakage. After locating the above leakage reason, the flange gasket structure is improved, and the bolt distribution and bolt preload is adjusted accordingly. The problem of leakage has been solved, approves that the calculating method of gasket sealing is an effective approach.

A numerical evaluation of next generation additive layer manufactured inter-layer channel heat exchanger

A Concept Heat Exchanger (HE) design manufactured using the Additive Layer Manufacturing (ALM) technique Selective Laser Melting (SLM) is proposed and numerically evaluated. It is composed of a HE corrugation which introduces inter-layer flow conduits between the parallel HE layers of the same fluid. These pathways are provided by hollow elliptical tubes which serve several functions: to disturb the flow to promote heat transfer, to provide additional heat transfer area and to minimise flow maldistribution inside the HE core. The corrugation is incorporated into a counter-flow prototype HE unit model meaning to exploit the installation volume and design freedom made possible via ALM. Three Computational Fluid Dynamics (CFD) models are utilised to evaluate the performance of the proposed HE unit. Firstly, a traditional two step HE design methodology is utilised which works by initially evaluating a fully symmetric channel of the proposed HE corrugation (termed single channel). Then the results this model are incorporated into a simplified HE unit model. The second approach evaluates the HE unit performance based on a fully detailed CFD analysis that fully resolves flow and heat transfer inside the HE core. The third modelling approach involves splitting the inter-layer HE unit model into parts, which results in HE header models and allows simplification of the HE core into a single corrugation period width HE core model (termed superchannel). The results of these models are then compared to a conventional pin–fin HE unit model, formed by blocking the elliptical inter-layer conduits. It was found that in all the HE unit models the pressure drop is similar whilst the heat transfer was enhanced by between 7% and 13% in terms of the overall ΔT by the inter-layer channels (increasing with the Reynolds number). All simulations were completed using a CFD package OpenFOAM.

Modeling framework to predict two-phase flow distribution in heat exchanger headers

Mini- and microchannel heat exchangers have the potential to drastically decrease the size and cost of thermal systems, but they often underperform because of flow maldistribution. Maldistribution can be particularly acute when a two-phase mixture enters a heat exchanger header, which is difficult to address because the flow phenomena in these situations are poorly understood. Experimental studies in recent years have attempted to gain a better understanding of the flow regimes and distribution in manifolds, but methods to predict distribution in multi-channel heat exchangers have been lagging. A new modeling approach, which predicts the liquid and vapor flow rates in each channel of a microchannel heat exchanger based on header flow patterns and component pressure drop, is presented here. The proposed model shows good agreement with previously reported experimental results, predicting most of the data within the experimental error.

Characterization of two-phase flow distribution in microchannel heat exchanger header for air-conditioning system

The two-phase flow distribution behavior of R410A within the vertical header of a microchannel heat exchanger with multiple horizontally oriented microchannel flat tubes was investigated and is reported in this paper. Unlike most previous studies, which examined the distribution at lower flowrates applicable mostly to automobile applications, this work evaluated higher flowrates relevant to actual air conditioning evaporator applications with larger size headers. The following operating conditions, were utilized: an inlet mass flowrate that varied from 40 to 200 kg h−1 (mass flux of 27–250 kg m−2 s−1 in the header), vapor qualities of 0.1, 0.2, and 0.6, and evaporating temperatures of 10 and 15 °C. The tube protrusion depth into the header was set at 0 and 50%. Flow distribution profiles derived from the experiment measurements and clear visualization images captured by a high speed camera showed that the distribution improves for increased inlet mass flux at low vapor quality, while a 5 °C difference in evaporating temperature does not yield a substantial distribution change. A 50% protrusion produces higher inertial forces pushing the liquid level towards the top section. A correlation was developed to predict the liquid distribution by relating the portion of liquid exiting the branch tube to the liquid at the immediate header as a function of the liquid Froude number.

Two-phase flow distribution of saturated refrigerants in microchannel heat exchanger headers

Mini- and microchannel heat exchangers have the potential to drastically decrease the size and cost of thermal systems, but they often underperform because of flow maldistribution. Maldistribution can be particularly acute when a two-phase mixture enters a heat exchanger header, which is difficult to address because the flow phenomena in these situations are poorly understood. In the present study, flow distribution in microchannel heat exchanger manifolds is investigated. The work focuses on two-phase flow regimes and distribution characteristics of saturated R134a in heat exchanger headers. The results quantify the effects of inlet mass flux (23.9 < Gin < 95.7 kg m−2 s−1), inlet quality (0.10 < xin < 0.90), header flow regime and header pressure drop on distribution. Additionally, several header geometries (rectangular, triangular, and vane) are investigated to identify pathways to mitigate maldistribution in heat exchangers.

Optimal design of heat exchanger header for coal gasification in supercritical water through CFD simulations

Abstract Heat exchangers play an important role in supercritical water coal gasification systems for heating feed and cooling products. However, serious deposition and plugging problems always exist in heat exchangers. CFD modeling was used to simulate the transport characteristics of solid particles in supercritical water through the shell and tube of heat exchangers to alleviate the problems. In this paper, we discuss seven types of exchangers (A, B, C, D, E, F and G), which vary in inlet nozzle configuration, header height, inlet pipe diameter and tube pass distribution. In the modeling, the possibility of deposition in the header was evaluated by accumulated mass of particles; we used the velocity contour of supercritical water (SCW) to evaluate the uniformity of the velocity distribution among the tube passes. Simulation results indicated that the optimum heat exchanger had structure F, which had a rectangular configuration of tube pass distractions, a bottom inlet, a 200-mm header height and a 10-mm inlet pipe diameter.

Optimization of the engine heat exchanger header with an external supply of heat

The paper makes hydraulic calculation of heat exchanger inlet manifold of engine with an external supply of heat.The estimation of uneven fluid flow through heat exchanger elements was made.

CFD simulation and optimization of fluid flow distribution inside printed circuit heat exchanger headers

ABSTRACTFluid flow distribution is related to the performance of heat exchangers. Gross flow distribution is often linked to conventional headers and thus jeopardizes the thermo-hydraulic performance. The objective of this paper is to design several configurations of printed circuit heat exchanger headers to enhance its flow uniformity while leveraging its etched nature. Four kinds of inlet headers, i.e., trapezoid header, double header with detail of bypass holes, core integrated header with one layer, and core integrated header with multilayers, are compared. It is found that the traditional headers induce poor flow distributions. Based on the flow characteristics in traditional headers, a novel core integrated header with seven merged channels is proposed. The flow nonuniformity of the heat exchanger with this novel inlet and outlet headers is reduced by 91%, but the pressure drop is increased by 114% compared with the baseline configuration, whereas it shows the best heat transfer performance as well due to the improvement of flow uniformity.

細径多列流路の折り返し合流部における冷媒二相流の流量分配特性

A flat tube heat exchanger has attracted attention as one of the solutions to improve energy efficiency and reduce the amount of refrigerant in the development of the air conditioning unit for global warming prevention. An experimental study is carried out to examine the heat exchanger header patterns capable of appropriately distributing gas-liquid two-phase refrigerant into several narrow channels in an inclined flat-tube heat exchanger return junction for the purpose of improving both condensation and evaporation heat transfer performance. First, distribution of the gas-liquid two-phase refrigerant are examined using a return junction model with lower four inlet tubes and upper four outlet tubes to obtain the slit pattern achieving a uniform distribution. Next, an improved slit pattern is examined to obtain a desired distribution rather than a uniform one. Furthermore, an S-shaped path which has two slits is proposed to improve both condensation and evaporation heat transfer performance, and its heat transfer performances is evaluated using the heat exchanger with the S-shaped path. As a result, the overall heat transmission coefficient of the flat-tube heat exchanger with the S-shaped path is improved by about 5% in evaporation and 10% in condensation compared to that of a heat exchanger without the slit.

A computational fluid dynamics and effectiveness-NTU based co-simulation approach for flow mal-distribution analysis in microchannel heat exchanger headers

Refrigerant flow mal-distribution is a practical challenge in most microchannel heat exchangers (MCHXs) applications. Geometry design, uneven heat transfer and pressure drop in the different microchannel tubes are three main reasons leading to the flow mal-distribution. To efficiently and accurately account for these three effects, a new MCHX co-simulation approach is proposed in this paper. The proposed approach combines a detailed header simulation based on computational fluid dynamics (CFD) and a robust effectiveness-based finite volume tube-side heat transfer and refrigerant flow modeling tool. The co-simulation concept is demonstrated on a ten-tube MCHX case study. Gravity effect and uneven airflow effect were numerically analyzed using both water and condensing R134a as the working fluids. The approach was validated against experimental data for an automotive R134a condenser. The inlet header was cut open after the experimental data had been collected. The detailed header geometry was reproduced using the proposed CFD header model. Good prediction accuracy was achieved compared to the experimental data. The presented co-simulation approach is capable of predicting detailed refrigerant flow behavior while accurately predicts the overall heat exchanger performance.

GS44 曲がりを伴う矩形管の急収縮急拡大損失特性

It is supposed that the pressure drop might become large when the heat exchanger headers are designed small in order to make the heat exchanger compact. This study discusses the test results on the pressure loss in the sudden contraction and expansion of the air and the water flow, when their elbow angles show 45 and 90 degrees in the rectangular pipe. This study also reports the new correlations among the concerning elements in the pressure drop coefficient.

Research on the Plastic Limit Pressure of the Plate-fin Heat Exchanger Header with Lateral Nozzle

To investigate the strength design of the plate-fin heat exchanger header with lateral nozzle,the calculation procedure of limit load for plastic work curvature(PWC) criterion by using ANSYS and Solidworks is presented,the limit pressure results of the header with 30 degree-lateral are compared for three limit pressure criteria,which are the PWC criterion,the twice elastic slope(TES) criterion and the American Society of Mechanical Engineers(ASME) limit load criterion of perfect plastic material,and the three dimensional parameters affecting the limit pressure,which are the lateral oblique angle β,the ratio of header diameter/header thickness iDδ,and the opening ratio idD,are studied.The comparison of limit pressure results for the header with 30 degree-lateral shows that the PWC limit pressure is characterized by reflecting the structural strengthening effect of the material strain-hardening on the header by using the plastic work and the plastic work curvature as the global indicators of plastic collapse mechanism,which is objective other than the arbitrary deformation parameter and the twice elastic slope failure line of TES criterion;the relationship study of limit pressure and the three dimensional parameters shows that the limit pressure is decreased with the smaller lateral oblique angle,larger opening ratio and larger header diameter/header thickness ratio.The research results can provide beneficial guideline for calculating the plastic limit pressure and selecting geometrical dimensional parameters in designing the plate-fin heat exchanger with lateral nozzle.