Dimensional analysis on the evaporation and condensation of refrigerant R-134a in minichannel plate heat exchangers

Two-phase flow analysis for the evaporation and condensation of refrigerants within the minichannel plate heat exchangers is an area of ongoing research, as reported in the literatures reviewed in this article. The previous studies mostly correlated the two-phase heat transfer and pressure drop in these minichannel heat exchangers using theories and empirical correlations that had previously been established for two-phase flows in conventional macrochannels. However, the two-phase flow characteristics within micro/minichannels may be more sophisticated than conventional macrochannels, and the empirical correlations for one scale may not work for the other one. The objective of this study is to investigate the parameters that affect the two-phase heat transfer within the minichannel plate heat exchangers, and to utilize the dimensional analysis technique to develop appropriate correlations. For this purpose, thermo-hydrodynamic performance of three minichannel brazed-type plate heat exchangers was analyzed experimentally in this study. These heat exchangers were used as the evaporator and condenser of an automotive refrigeration system where the refrigerant R-134a flowed on one side and a 50% glycol–water mixture on the other side in a counter-flow configuration. The heat transfer coefficient for the single-phase flow of the glycol–water mixture was first obtained using a modified Wilson plot technique. The results from the single-phase flow analysis were then used in the two-phase flow analysis, and correlations for the refrigerant evaporation and condensation heat transfer were developed. Correlations for the single-phase and two-phase Fanning friction factors were also obtained based on a homogenous model. The results of this study showed that the two-phase theories and correlations that were established for conventional macrochannel heat exchangers may not hold for the minichannel heat exchangers used in this study.     

Frictional Pressure Drop for Gas/Liquid Two-Phase Flow in Plate Heat Exchangers

Two-phase flow occurs in plate heat exchangers used as evaporators or condensers. The frictional pressure drop for two-phase flow can be considerably higher than for single-phase flow, because of the interaction between the phases. In this article, measurements of air-water flow in two different plate-and-frame exchangers are presented. Correlations developed for tubular flow are compared with the experimental results. Recommendations for a calculation procedure are given.     

Thermo-Hydraulic Performance Enhancement of Finned Elliptical Tube Heat Exchangers by Utilizing Innovative Dimple Turbulators

Abstract

As a new type of fin structure in finned tube heat exchangers, dimple turbulators exhibit excellent potential for thermo-hydraulic performance enhancement. A three-dimensional numerical simulation study was conducted to investigate the influences of five kinds of innovative concave dimple turbulators (CDTs), namely – elliptical dimple, conical frustum dimple, trapezoidal prism dimple, leeward triangular dimple and upward triangular dimple (UwTD) on the thermo-hydraulic performance enhancement in a plate fin-and-elliptical tube (PFET) heat exchanger, where CDTs are textured on the fin surface transversely between the elliptical tubes. The computational results are analyzed by considering the performance evaluation criterion for the PFET heat exchangers with different types of CDT shapes. The present investigation demonstrates that the heat transfer enhancement is intimately pertained to ejection with longitudinal counter-rotating flow, strengthened secondary flow and vortex structures at the downstream rim of CDT. A parametric study on the CDTs indicated that the UwTD vortex turbulators give better thermo-hydraulic performance under the present conditions. The numerical simulation results illustrated different secondary flow structures and heat transfer characteristics of the CDTs with various shapes, which disclosed the influential mechanisms of differently shaped dimple turbulators on the heat transfer augmentation in PFET heat exchangers.     

A general calculation method for plate heat exchangers

The possibilities of process stream guidance within plate heat exchangers are very complex. There exist constructional solutions, in which the process-streams are led through stream-passages connected in series or parallel. Other constructions realize the heat transfer between more than two process streams. Known are also constructions, where the stream passages are spirals. All these constructions deviate more or less from the ideal principle of uniflow or counterflow guidance. While the approximate interpretation of plate heat exchangers is solved methodically, the only methods used for the calculation of existing apparatus are either mathematically extremely exacting or not generally valid. To analyse the behaviour of plate heat exchangers, exact and easy to apply calculation methods are required. This article presents a general calculation method for the stationary simulation of plate heat exchangers of any type required. The method allows a mathematically exact determination of the temperature profiles of types of apparatus mentioned above. In a further step the proposed method can also be used for the simulation of the heat flux along the walls of the plates as well as the dispersion in the passages. The basic idea of the presented method is the simulation of a heat exchanger network with generalized operating characteristics. The “classic formulas” of this operating characteristic are given in this method as special cases. The general calculation method for plate heat exchangers can be used without any iteration. Only the consideration of temperature dependent properties as well as phase transitions require iterations. This method is characterized by a great numerical stability.     

The Simulation of Multicomponent Mixtures Condensation in Plate Condensers

The application of plate heat exchangers for the condensation of multicomponent mixtures requires reliable, well-grounded methods of calculation. A numerical simulation using semi-empirical equations of heat and mass transfer performance along the surface of plate condensers was carried out for different multicomponent mixtures with noncondensable components. The plates with cross-corrugated patterns for plate condensers were used. The simulation was done for four different types of corrugated plates of industrially manufactured plate heat exchangers.

The results of the simulation are in a good accordance with experimental data obtained during long-time experiments for a pilot plant at the pharmaceutical factory in Kharkiv.

It is shown that the enhancement of heat and mass transfer in a plate condenser for the case of a four-component mixture gives the possibility of decreasing by 1.8–2 times the necessary heat transfer surface area comparatively with shell-and-tube unit for the same process parameters.     

扭带强化管内单相对流换热的研究进展

能源与材料费用的不断增长推进了高效节能换热器的发展.在管壳式换热器中,为强化管内单相对流换热并有效清除污垢,制造工艺简单、拆装维修方便的管内插入物-扭带被广泛应用.介绍了多种扭带的结构和特点,分析了扭带强化管内单相对流换热的机理.从连续扭带、闻隔扭带、异型扭带、清洗扭带和复合强化技术五个方面,对扭带强化管内单相对流换热...     

Thermal evaluation of nanofluids in heat exchangers

Nanofluids, suspensions of nanoparticles (less than 100 nm) in a basefluid, have shown enhanced heat transfer characteristics. In this study, thermal performances of nanofluids in industrial type heat exchangers are investigated. Three mass particle concentrations of 2%, 4%, and 6% of silicon dioxide–water (SiO₂–water) nanofluids are formulated by dispersing 20 nm diameter nanoparticles in distilled water. Experiments are conducted to compare the overall heat transfer coefficient and pressure drop of water vs. nanofluids in laboratory-scale plate and shell-and-tube heat exchangers. Experimental results show both augmentation and deterioration of heat transfer coefficient for nanofluids depending on the flow rate and nanofluid concentration through the heat exchangers. This trend could be explained by the counter effect of the changes in thermo-physical properties of fluids together with the fouling on the contact surfaces in the heat exchangers. The measured pressure drop while using nanofluids show an increase when compared to that of basefluid which could limit the use of nanofluids in industrial applications.     

Heat Transfer and Flow Characteristics of Al2O3/Water Nanofluid in Various Heat Exchangers: Experiments on Counter Flow

Abstract

On account of nanofluids influence on heat exchangers (HEs), a vigorous discussion can be made to concurrently contrast HEs to one another under the same conditions to detect maximum efficacy. Based on an extensive experimental study, this research is established to examine the effect of nanofluids on the performance of heterogeneous HEs with the same heat transfer surface area considering counter flow arrangement. A double pipe HE, a shell and tube HE and a plate HE are intended to accomplish the experiments. The experiments are executed under turbulent flow conditions using distilled water and Al₂O₃/water nanofluid with 0.2, 0.5, and 1% particle volume concentrations. From the results shown in the article, the double pipe HE revealed the best outcome for the heat transfer coefficient with a maximum enhancement of 60% while a maximum enhancement in the heat transfer coefficient of 11% was reported for the plate HE. Utilizing a nanofluid represented the lowest penalty in the pressure drop with a maximum enhancement of 27% for the plate HE while the highest penalty in the pressure drop with a maximum enhancement of 85% was observed in the double pipe and shell and tube HEs.     

Thermofluid characteristics of frosted finned-tube heat exchangers

The performance of frosted finned-tube heat exchangers of different fin types is investigated by experiments in this paper. The effects of the air flow rate, the air relative humidity, the refrigerant temperature, and the fin type on the thermofluid characteristics of the heat exchangers are discussed. The time variations of the heat transfer rate, the overall heat transfer coefficient, and the pressure drop of the heat exchangers are presented. The heat transfer rate, the overall heat transfer coefficient, and the pressure drop for heat exchangers with re-direction louver fins are higher than those with flat plate fins and one-sided louver fins are. The amount of frost formation is the highest for heat exchangers with re-direction louver fins.     

等雷诺数法在板式换热器传热试验中的应用

在换热器的传热性能试验中,对流换热系数的测定是重要的一个组成部分,对流换热系数的测定方法有很多种,但都有各自的应用范围和条件。对于板式换热器而言,用等雷诺数法来获得其对流换热系数是比较合适的一种方法,文中介绍了等雷诺数法的原理及其在板式换热器中的应用。通过实例,介绍了具体的试验及计算方法。     

Steady-State and Transient Behavior of Two Heat Exchangers Coupled by a Circulating Flowstream

Systems consisting of two heat exchangers coupled by a circulating flowstream are studied. The systems differ in the flow configurations of the single heat exchangers. For steady-state operation, there exists a heat capacity rate of the circulating flowstream that maximizes the temperature changes of the external flowstreams. Until now, this optimum has been calculated assuming that the overall heat transfer coefficients of the heat exchangers do not depend on the mass flow rate of the circulating flowstream. In this paper, the dependence of the overall heat transfer coefficient on the mass flow rate of the circulating flowstream is taken into account. For transient operating conditions, the system response to perturbations of inlet temperatures and mass flow rates is calculated by the method of Laplace Transforms and an explicit finite difference method. The most significant features of the coupled system become apparent by considering outlet temperature transients induced by perturbations of the mass flow rate of the circulating flowstream.     

Transport Phenomena and Chemical Reactions in Modular Microstructured Devices

ABSTRACT

Fluid flow and heat transfer in microchannels is since decades a wide area of vivid research in electronics cooling, compact heat exchangers, or chemical analytics. Fluid dynamics, mixing, and heat transfer are very important for performing exothermic chemical reactions in microchannels under stable and robust conditions. This contribution describes the complex interplay of single and multi-phase flow, residence time, mixing, and heat transfer with chemical kinetics. Dimensionless numbers and typical time scales guide the appropriate design and operation of microstructured equipment and chemical processes. Well-characterized, modular equipment enable short cut methods in the conceptual design phase and allow for consistent scale-up to production scale. The complex correlations are displayed here in a simple fashion with arrows, which can be replaced by relevant equations. The equipment is categorized with a toolbox concept of plate, tube, and vessel on certain platform levels for similar volumetric throughput and process conditions. In addition, separation units are presented for complete chemical process design on laboratory scale. This toolbox enables consistent process development and scale up to small-scale production ranges.     

Non-adiabatic capillary tube flow: a homogeneous model and process description

This paper presents a homogeneous model of refrigerant flow through capillary tube–suction line heat exchangers, which are widely used in small vapour compression refrigeration systems. The homogeneous model is based on fundamental conservation equations of mass, momentum and energy. These equations are solved simultaneously through iterative process. Churchill’s correlation [3] is used to calculate single-phase friction factors and Lin et al. [6] correlation for two-phase friction factors. The single-phase heat transfer coefficient is calculated by Gnielinski’s equation [5] while two-phase flow heat transfer coefficient is assumed to be infinite. The model is validated with previous experimental and analytical results. The present model can be used in either design calculation (calculate the capillary tube length for given refrigerant mass flow rate) or simulation calculation (calculate the refrigerant mass flow rate for given capillary tube length). The simulation model is used to understand the refrigerant flow behaviour inside the non-adiabatic capillary tubes.     

Heat Transfer Characteristics of a Quadratic Koch Island Fractal Heat Exchanger

Heat exchangers are widely used in the air conditioning and refrigeration industries, and any increase in their efficiency will have a positive effect on the industry as well. A new design of heat exchanger is proposed that will increase the heat transfer area significantly. The heat transfer area was increased by the use of fractals. Three techniques were used to investigate the heat transfer increase; analytical, numerical, and experimental methods. The results showed that the fractal heat exchanger has a higher heat transfer to overall volume ratio than a conventional tube-in-tube heat exchanger.     

Effect of Flow Distribution to the Channels on the Thermal Performance of the Multipass Plate Heat Exchangers

Over last two decades, plate heat exchangers (PHEs) have presented themselves as a viable alternative to the conventional shell and tube heat exchangers in the process and power industries. The thermal theory available for plate heat exchangers in the literature largely works on the assumption of equal flow in each channel. However, it is well known that the distribution of fluid from port to channel in PHE is far from being uniform. The present study brings about this port to channel flow distribution effect on the thermal behavior of multipass plate heat exchangers. The variation of the heat transfer coefficient due to flow variation from channel to channel has also been taken into consideration. Heat exchangers with both equal and unequal passes of the fluids have been studied. The results indicate that the flow maldistribution severely affects the performance of plate heat exchangers, and multipassing can act as an important tool to reduce the deterioration in performance due to maldistribution. The results show that with a low number of passes, the increase of velocity of fluid may be counterproductive in terms of heat transfer enhancement. Also, adding plates in order to increase the heat transfer surface may not be effective due to an increase in flow maldistribution. The correlations for 1-1, 1-2, 2-2, and 2-3 pass plate heat exchangers with the maldistribution index as a parameter are also presented.     

Exergy—Economic Criteria for Evaluating Heat Exchanger Performance

honductiOnHeat exchangee as one of the thennal equlpment,is widely used in energy systems, the imPrvement ofheat exchangers perfonnance is imPOrtan for savingenergy and twving tbe efficiency, so it is necessaryto emPloy a general driterion to evaluate heatexchanger perfrmance. In the design of heatexchangers, numrous perfrmance evaluation criteriaealst tO heIP to detennine their fhatUfs11]. Thesecriteria lead towards the goal of oPtindzationprocedurs. They dePend on the asPeCt of o~zation…     

A new approach for thermal performance calculation of cross-flow heat exchangers

A new numerical methodology for thermal performance calculation in cross-flow heat exchangers is developed. Effectiveness-number of transfer units (ε-NTU) data for several standard and complex flow arrangements are obtained using this methodology. The results are validated through comparison with analytical solutions for one-pass cross-flow heat exchangers with one to four rows and with approximate series solution for an unmixed-unmixed heat exchanger, obtaining in all cases very small errors. New effectiveness data for some complex configurations are provided.     

Research Needs in Low Reynolds Number Flow Heat Exchangers

Low Reynolds number flows prevail in some process, power, automotive, aircraft, and other industrial heat exchangers such as shell-and-tube, plate, and compact heat exchangers. Laminar or low Reynolds number turbulent flows are the results of either high viscosity fluids, compact flow passages (i.e., small hydraulic diameter), or low fluid velocities. Significant advancements have been made in the past 100 years in understanding and predicting flow and heat transfer in such internal flows. This paper summarizes the research needs to further advance the science of low Reynolds number flow heat exchangers. Emphasis is primarily given to the thermal design aspects; research needs related to mechanical design, manufacturing, material selection, and other nonthermal design aspects are not covered. Also the coverage is restricted primarily to single-phase applications. The outlined research needs are based on the input from invited lecturers and some participants, experts in the field, at the Fourth NATO Advanced Study Institure in Ankara, Turkey, July 1981.     

Enhancement of Heat Transfer for Plate Fin Heat Exchangers Considering the Effects of Fin Arrangements

In the present study, the potential of rectangular fins with different fin types of inner zigzag-flat-outer zigzag (B-type) and outer zigzag-flat-outer zigzag (C-type) and with different fin angles of 30° and 90° for 2 mm fin height and 10 mm offset from the horizontal direction for heat transfer enhancement with the use of a conjugated heat transfer approach and for pressure drop in a plate fin heat exchanger is numerically evaluated. The rectangular fins are located on a flat plate channel (A-type). The numerical computations are performed by solving a steady, three-dimensional Navier–Stokes equation and an energy equation by using FLUENT software program. Air is taken as working fluid. The study is carried out at Reynolds number of 400 and inlet temperatures, velocities of cold and hot air are fixed as 300 K, 600 K and 1.338 m.s^?1, 0.69 m.s^?1, respectively. This study presents new fin geometries which have not been researched in the literature for plate fin heat exchangers. The results show that while the heat transfer is increased by about 10% at the exit of a channel with the fin type of C, it is increased up to 8% for the fin angle of 90° when compared to a channel with A-type under the counter flow. The heat transfer enhancements for different values of Reynolds number and for varying fin heights, fin intervals and also temperature distributions of fluids are investigated for parallel and counter flow.     

开缝布置方式对开缝翅片管换热器 传热与阻力特性的影响

为了获得开缝布置方式对开缝翅片管换热器传热与阻力特性的影响规律,对5种不同翅片管换热器进行了数值模拟研究,并进行了模化试验验证。结果表明：增加开缝会提高翅片管换热器的传热性能,但阻力也随之增加;与开缝位置相比,开缝数量对开缝翅片管换热器传热与阻力特性的影响更大;在Re=4800~7500日时,开缝翅片管换热器综合流动传热性能 随着Re数的增大而增大;在5种翅片中,开缝翅片的综合流动传热性能高于普通平直翅片;数值模拟与试验结果偏差较小,采用数值模拟方法能够比较准确地分析开缝翅片管换热器的传热与阻力特性。