Parallel Flow Heat Exchanger Research Articles

Characteristics of flow distribution in compact parallel flow heat exchangers, part I: Typical inlet header

This study experimentally and numerically investigates the single-phase flow into parallel flow heat exchangers with inlet and outlet rectangular headers having square cross section and 9 circular tubes. The effects of inlet flow condition, tube diameter, header size, area ratio, flow directions (Z and U-type), as well as the gravity are investigated. The experimental results indicate that flow distribution for U-type flow is more uniform than Z-type flow. Depending on the inlet volumetric flow rate, the flow ratio at the first several tubes can be lower than 50% of the last tube for Z-type arrangement, and this phenomenon becomes more and more pronounced with the rising velocity at the intake conduit. The mal-distribution can be eased via reducing the branching tube size or increasing the entrance settling distance at the intake conduit. It is found that the influence of gravity on mal-distribution is negligible and the mal-distribution is associated with the jet flow pattern.

Analysis of a Parallel-Flow Heat Exchanger with a Heat Source

In this work, a modified analysis of a parallel-flow plate heat exchanger that takes into account a volumetrically uniform heat source in the hot fluid is presented. New expressions for the number of transfer units (NTU) and effectiveness of the heat exchanger are derived. These expressions are verified against the conventional effectiveness–NTU relations in the limit of zero heat source rate. This situation is of interest in applications such as the ammonia–water absorption absorber heat exchanger where a heat source is generated in the solution side. The model studies two cases based on the minimum and maximum heat capacities of the hot fluid. The results show that the number of transfer units and the effectiveness of the heat exchanger are the same for both cases. The analysis is applied to the absorber heat exchanger. Expressions of effectiveness and number of transfer units of a counterflow heat exchanger with a heat source in the hot fluid stream are also given from minimum and maximum heat capacities points of view.

Refrigerant distribution in a parallel flow heat exchanger having vertical headers and heated horizontal tubes

Refrigerant R-410a flow distribution is experimentally studied in a test section simulating a parallel flow heat exchanger having vertical headers with two pass configuration. Tubes are heated to yield a test section outlet superheat of 5 °C with inlet quality of 0.3. Mass flux is varied from 50 kg/m 2 s to 70 kg/m 2 s. Effects of inlet and outlet locations are investigated in a search for an optimum configuration. Results show that, significant liquid flows through bottom channels, and less liquid is supplied to top channels. As for the inlet location, better flow distribution (pressure drop as well) is obtained for top inlet as compared with middle inlet. As for the outlet location, top or bottom outlet is better than middle outlet. Correlations are developed for the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream. The correlations may be used to predict the liquid or gas distribution in a parallel flow heat exchanger having vertical headers. A novel thermal performance evaluation method, which accounts for tube-side flow mal-distribution is proposed.

EFFECTS OF BUOYANCY ON THE PERFORMANCE OF A VERTICAL DOUBLE-PIPE HEAT EXCHANGER

CFD simulations of heat transfer between two fluids in a parallel-flow vertical double-pipe heat exchanger, taking into consideration conduction in the walls, viscous dissipation, variable fluid properties, and axial diffusion in the fluids, illustrate the effects of buoyancy on the velocity and temperature profiles as well as on the Nusselt numbers and friction coefficients. In particular, the results show that flow reversal occurs at different positions depending on the flow direction. In the developing region, the fluid bulk temperatures, the wall temperature, and the local Nusselt numbers for both fluids depend on the flow direction. For the conditions under consideration, the Nusselt number for the fully developed flow in the inner pipe lies between the corresponding forced convection values for the constant heat flux and constant temperature conditions, while the one in the annulus is smaller than both these forced convection values. It is established that the flow and thermal fields in the inner cylinder and the annulus cannot be predicted correctly by studying each one separately since the thermal condition at the solid wall separating the two fluids depends on flow characteristics such as the Richardson number in the inner tube.

バックステッピング法を用いた拡散項を有する並流型熱交換方程式の出力フィードバック制御

This paper is concerned with the problem of enhancing the stability of a parallel-flow heat exchanger equation with diffusive terms, under boundary control and boundary observation. By introducing two new variables, the system is expressed as two decoupled subsystems, where each subsystem is exponentially stable. In order to enhance the stability of the whole system, we use a backstepping method to construct infinite-dimensional observer-based controllers for each subsystem. The Lyapunov method is successfully applied to prove the closed-loop stability with the controller. This paper gives the controller design for the case of collocated sensor and actuator as well as for the anticollocated case.

EFFECT OF INLET CONFIGURATION ON DISTRIBUTION OF AIR–WATER UPWARD FLOW IN A HEADER OF A PARALLEL FLOW HEAT EXCHANGER

The effect of inlet configuration (parallel, normal, vertical) on flow distribution in a parallel flow heat exchanger consisting of round headers and ten flat tubes is experimentally studied using air and water. The effects of tube protrusion depth as well as header mass flux, and quality are investigated for upward flow configuration. It is shown that best flow distribution is obtained for vertical inlet configuration, followed by normal inlet and parallel inlet configuration. For upward flow, significant portion of the water flows through the rear part of the header. As protrusion depth increases, more water is forced to the rear part of the header. The effect is most significant for parallel inlet, followed by normal and vertical inlet. The effect of mass flux or quality is opposite to that of the protrusion depth. Possible explanation is provided from flow visualization results.

Thermal analysis of a counterflow heat exchanger with a heat source

SYNOPSIS A modified analysis is presented of a counterflow plate heat exchanger that takes into account a heat source, which is assumed to be volumetrically uniform in the hot fluid. New expressions for the number of transfer units (NTU) and effectiveness of the heat exchanger are derived. These expressions are verified against the conventional effectiveness-NTU relationships in the limit of zero heat source rate. Such heat exchanger with heat source in the hot fluid is of interest in applications to ammonia-water absorption heat exchanger where a heat of absorption source term arises in the solution side. The model studies two cases based on the minimum and maximum heat capacities of the hot fluid. The results show that the number of heat transfer units and the effectiveness of the heat exchanger are not the same for both cases. The analysis is applied to an absorber heat exchanger. Effectiveness-NTU relationships are also given for a parallel flow heat exchanger with a heat source in the hot fluid.

Data-driven fuzzy models for nonlinear identification of a complex heat exchanger

This paper presents and discusses experimental results on nonlinear model identification method applied to a real pilot thermal plant. The aim of this work is to develop a moderately complex model with interpretable structure for a complex parallel flow heat exchanger which is the main component of the thermal plant using a fuzzy clustering technique. The proposed Takagi–Sugeno-type (TS) fuzzy rule-based model is derived through an iterative fuzzy clustering algorithm using a set of input–output measurements. It is shown that the identified multivariable fuzzy rule-based model captures well the key dynamical properties of the physical plant over a wide operating range and under varying operating conditions. For validation, the model is run in parallel and series-parallel configurations to the real process. The experimental results show clearly the high performance of the proposed fuzzy model in achieving good prediction of the main process variables.

Boundary control of a parallel-flow heat exchanger by input–output linearization

A design of the control of the internal fluid temperature at the outlet of a parallel-flow heat exchanger by manipulating the inlet external fluid temperature is proposed. The dynamic model of the heat exchanger is given by two partial differential equations that are used without spatial discretization to design the control law. Based on nonlinear control, a state-feedback law that ensures a desired performance of a measured output defined as the spatial weighted average temperature of the internal fluid is derived. Then, in order to control the outlet internal fluid temperature, a control strategy is proposed where an external controller is introduced to provide the set point of the considered measured output by taking as input the error between the outlet internal fluid temperature and its desired set point. As the designed control law is a state feedback of distributed nature, for practical application, a Kalman filter is used to reconstruct the entire state of the system from the measurements of the outlet fluids temperatures. The closed-loop system is shown to be exponentially stable. The validity of the proposed control design is examined in simulation by considering the tracking and perturbation rejection problems.

Dialysis in parallel-flow rectangular membrane modules with external reflux for improved performance

The effect of external recycle on the performance of dialysis in countercurrent-flow rectangular membrane modules was investigated both theoretically and experimentally. Theoretical analysis of mass transfer in parallel-flow device with and without recycle is analogous to heat transfer in parallel-flow heat exchangers. Experiments were carried out with the use of a microporous membrane to dialyze urea aqueous solution by pure water. In contrast to a device with recycle, improvement in mass transfer is achievable if parallel-flow dialysis is operated in a device of same size with recycle which provides the increase of fluid velocity, resulting in reduction of mass-transfer resistance, especially for rather low feed volume rate.

Observability of the two-stream parallel-flow heat exchanger equation

In this paper, we study the observability of the two-stream parallel-flow heat exchanger equation with boundary inputs and two temperature measurements. For a class of initial values and inputs, we obtain the exact observability. It is also shown that the system becomes unobservable if only one fluid temperature is measured.

The Effect of Fouling on Low Temperature Heat Exchangers Performance Based on Exergy Analysis

By analyzing heat and exergy transfer processes of heat exchangers operating below the surrounding temperature, the effect of fouling on the low temperature heat exchangers performance has been conducted. Taking the parallel flow, counter flow, and cross flow low temperature heat exchangers as examples, the effects of some non-dimensional parameters on the low temperature heat exchangers performance under the fouling condition are given by introducing exergy transfer units number.

Entransy dissipation number and its application to heat exchanger performance evaluation

Based on the concept of the entransy which characterizes heat transfer ability, a new heat exchanger performance evaluation criterion termed the entransy dissipation number is established. Our analysis shows that the decrease of the entransy dissipation number always increases the heat exchanger effectiveness for fixed heat capacity rate ratio. Therefore, the smaller the entransy dissipation number, the better the heat exchanger performance is. The entransy dissipation number in terms of the number of exchanger heat transfer units or heat capacity rate ratio correctly exhibits the global performance of the counter-, cross- and parallel-flow heat exchangers. In comparison with the heat exchanger performance evaluation criteria based on entropy generation, the entransy dissipation number demonstrates some distinct advantages. Furthermore, the entransy dissipation number reflects the degree of irreversibility caused by flow imbalance.

STATE MODEL OF STEAM REHEATING FOR CONTROL OPTIMIZATION

This paper presents a nonlinear state space model for thermal process of steam reheating. The purpose of the model is to examine existing control system and to support the development of new more efficient control systems for steam reheating units. This model describes countercurrent heat exchanger (biflux) with bypass and two parallel flow heat exchangers with bypasses and spray attemperator. The model includes interconnections of subsystems and the dynamics of all control valves. The model is a state model which enables modeling of power variation from start up to shut down. First, the basic principles of this approach are compared with already published results. Second, the model is calibrated for the newly designed reheating unit for the planned upgrade of a power plant. Both results from static and dynamic characteristic responses, as well as selected control responses in operating range are presented.

Drop-in Test of Microchannel Evaporators Using R-22 for Residential Air Conditioners

The drop-in test of microchannel evaporators using R-22 for residential air conditioners was conducted. Five prototype microchannel evaporators were manufactured and tested in psychrometric calorimeter test facilities. Each evaporator was made with two parallel flow heat exchangers connected with several return pipes. The parallel flow heat exchanger had 41 microchannel tubes with eight rectangular ports. Performance comparisons were made between microchannel evaporators with and without hydrophilic coating to assess the effect on condensate on evaporator surface. Also, various performances of prototype microchannel evaporators were presented. Note that the cooling capacities of prototype 2, with a flow area ratio of 73/58%, and prototype 3, with six return pipes, were larger than prototype 1 by 15% and 9%, respectively. The cooling capacity of prototype 4 with three rows was larger than prototype 1 by 14%. Both refrigerant-side and air-side pressure drops for prototype 2 with the best cooling capacity were 29 kPa and 2.5 mmAq, which are acceptable for residential air conditioners.

Numerical analysis of mass transfer in double-pass parallel-plate dialyzers with external recycle

The effect of external recycle on the performance of dialysis in double-pass rectangular membrane modules was investigated. Theoretical analysis of mass transfer in parallel-flow dialyzers is analogous to heat transfer in parallel-flow heat exchangers. In contrast to a single-pass device without recycle, improvement in mass transfer of about 30% is achievable if parallel-flow dialysis is operated in a double-pass device of same size with external recycle which provides the increase of fluid velocity, resulting in reduction of mass-transfer resistance.

Wet surface heat transfer and pressure drop of aluminum parallel flow heat exchangers at different inclination angles

The effect of inclination angle on the heat transfer and pressure drop characteristics of brazed aluminum heat exchangers was experimentally investigated under wet conditions. Three samples having different fin pitches (1.25, 1.5 and 2.0 mm) were tested. Results show that heat transfer coefficients are not affected by the inclination angle. However, friction factors increase as the inclination angle increases with negligible difference between the forward and backward inclination. The effect of fin pitch on the heat transfer coefficient and on the pressure drop is also discussed. Comparison of the dry and wet surface heat transfer coefficients reveals that dry surface heat transfer coefficients are significantly larger than wet surface heat transfer coefficients. Possible explanation is provided by considering the condensate drainage pattern. The data are also compared with the existing correlation.

Effectiveness-NTU Relations for Parallel Flow Heat Exchangers Subjected to Heat Leak from Outside

The effectiveness-NTU relations are a valuable piece of information for designing and rating of heat exchangers when heat leaks are significant. In this paper, the closed-form relations for parallel flow heat exchangers when the heat leak is either on the cold or hot side of the heat exchanger are presented. The results are also presented in a graphical form to illustrate dependence of the fluid capacity ratio as well as the dimensionless heat leak parameter. It is demonstrated that when the dimensionless heat leak term approaches zero, the solution reduces to the classical effectiveness-NTU relations for parallel-flow configurations. In addition, when the ratio approaches zero, the analytical solutions reduce to the cases of condenser and evaporator applications.

Performance comparison of microchannel evaporators with refrigerant R-22

An experimental study on the performance comparison of microchannel evaporators with refrigerant R-22 was conducted. Six microchannel evaporators were designed and manufactured for a residential air-conditioner. They were tested with psychrometric calorimeter test facilities. The experiment was performed with both vapor compression system and refrigerant circulation system. Each evaporator was made up of two parallel flow heat exchangers connected with several return pipes. The parallel flow heat exchanger had 41 microchannel tubes inserted between inlet and outlet headers. The microchannel tube had 8 rectangular ports with the hydraulic diameter of 1.3 mm. For the vapor compression system, the flow area ratio and the number of return pipes had a great effect on the cooling capacity. Type 3 with a flow area ratio of 73% and 58% showed the best cooling capacity. It had 12 return pipes and 3 circuits. There is a merging manifold in it. The effect of the number of circuits and merging manifold on the cooling capacity was relatively small. For the refrigerant circulation system, the effect of the mass flow rate on the cooling capacity was slightly superior to that of inlet quality. The effect of the number of circuits on the cooling capacity was different from the result of the vapor compression system. The effect of merging manifold was negligible, which was consistent with the result of the vapor compression system. The cooling capacity proportionally increased as the vertical inclination angle of the evaporator increased due to gravity force.

Distribution of air–water annular flow in a header of a parallel flow heat exchanger

The air and water flow distribution are experimentally studied for a heat exchanger composed of round headers and 10 flat tubes. The effects of tube protrusion depth as well as header mass flux, and quality are investigated, and the results are compared with previous 30 channel data. The flow at the header inlet is annular. For the downward flow configuration, water flow distribution is significantly affected by tube protrusion depth. For flush-mounted geometry, significant portion of water flows through frontal part of the header. As the protrusion depth increases, more water is forced to rear part of the header. The effect of header mass flux or quality is qualitatively the same as that of the protrusion depth. For the upward flow configuration, however, significant portion of water flows through rear part of the header. The effect of protrusion depth is the same as that of the downward flow. However, the effect of header mass flux or quality is opposite to the downward flow case. Compared with the previous 30 channel configuration, the present 10 channel configuration yields better flow distribution. Possible explanation is provided from flow visualization results.