Heat transfer coefficients under dry- and wet-surface conditions for a spirally coiled finned tube heat exchanger

In the present study, the average tube-side and air-side heat transfer coefficients in a spirally coiled finned tube heat exchanger under dry- and wet-surface conditions are experimentally investigated. The test section is a spiral-coil heat exchanger, which consists of six layers of concentric spirally coiled tube. Each tube is fabricated by bending a 9.6-mm outside diameter straight copper tube into a spiral coil of four turns. Aluminium fins with thickness 0.6 mm and outside diameter 28.4 mm are placed helically around the tube. The chilled water and the hot air are used as working fluids. The test runs are done at the air and water mass flow rates ranging between 0.02 and 0.2 kg/s and between 0.04 and 0.25 kg/s, respectively. The inlet-air and -water temperatures are between 35 and 60 °C and between 10 and 35 °C, respectively. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. New correlations based on the data gathered during this work for predicting the tube-side and air-side heat transfer coefficients for the spirally coiled finned tube heat exchanger are proposed.     

Thermal performance and pressure drop of the helical-coil heat exchangers with and without helically crimped fins

In the present study, the thermal performance and pressure drop of the helical-coil heat exchanger with and without helical crimped fins are studied. The heat exchanger consists of a shell and helically coiled tube unit with two different coil diameters. Each coil is fabricated by bending a 9.50 mm diameter straight copper tube into a helical-coil tube of thirteen turns. Cold and hot water are used as working fluids in shell side and tube side, respectively. The experiments are done at the cold and hot water mass flow rates ranging between 0.10 and 0.22 kg/s, and between 0.02 and 0.12 kg/s, respectively. The inlet temperatures of cold and hot water are between 15 and 25 °C, and between 35 and 45 °C, respectively. The cold water entering the heat exchanger at the outer channel flows across the helical tube and flows out at the inner channel. The hot water enters the heat exchanger at the inner helical-coil tube and flows along the helical tube. The effects of the inlet conditions of both working fluids flowing through the test section on the heat transfer characteristics are discussed.     

基于Fluent螺旋槽管的传热特性数值模拟

根据螺旋槽管的结构特点及传热特性,建立了三种不同槽口形状的螺旋槽管与光滑管换热器的三维模型。以水为工质,运用 Fluent流体分析软件,采用k-ε湍流模型,研究了三种不同槽口形状的螺旋槽管与光滑管换热器在换热过程中的速度场和温度场,得到了不同槽口形状和光滑管的壁面Nusselt数。结果表明。在相同壳程和雷诺数的情况下,螺旋槽管比光滑管的换热能力提高了6．7％-37．6%,其中三角彤槽和矩形槽螺旋槽管的换热能力提高最大,从而强化了传热。为谊产品的理论进一步研究和实验研究奠定了基础,为谊产品的设计和推广应用提供了依据。     

Single-phase heat transfer and pressure drop in the micro-fin tubes with coiled wire insert

The heat transfer characteristics and the pressure drop of the horizontal double pipes with and without coiled wire insert are investigated. The inner and outer diameters of the micro-fin tube are 8.92 and 9.52 mm, respectively. The coiled wire is fabricated by bending a 1-mm-diameter iron wire into the coil wire with coil diameter of 7.80 mm. Cold and hot water are used as working fluids in shell side and tube side, respectively. The test runs are performed at the cold and hot water mass flow rates ranging between 0.01 and 0.07 kg/s and between 0.04 and 0.08 kg/s, respectively. The inlet cold and hot water temperatures are between 15 and 20 °C and between 40 and 45 °C, respectively. The results obtained from the micro-fin tube with coiled wire insert are compared with those obtained from the smooth and micro-fin tubes.     

管内插入扭带及螺旋线圈的传热与阻力特性实验研究

实验研究了以空气为工质的管内插入扭带与螺旋线圈的传热与阻力特性,在3000相似文献   

Study of the Effect of Fin-and-Tube Heat Exchanger Fouling on its Structural Performance

This paper presents the thermal and structural analysis of high temperature fin-and-tube heat exchanger. Water flowing in tubular space and flue-gas flowing in the intertubular space, were considered as working fluids. The effect of limescale fouling on thermal and structural performance of heat exchanger was studied. The analysis considered an industrial heat exchanger, which failure occur from time to time. The expert inspection, after the failure indicated the existence of fouling layer within the heat exchanger tubes. In order to understand the reasons of heat exchanger failure, a detailed fluid flow analysis (both in the tubular and intertubular spaces) was performed. The analysis indicated that the silicate limescale fouling layer with thermal conductivity of 0.35 W/(mK) and thickness up to 1.5 mm existing in the tube, may increase the tube wall temperature even more than 150°C. The study also includes the impact of outer tube wall surface fouling with thickness of 0.2 mm and heat transfer coefficient of 2 W/(mK). As a result, the compressible stresses may increase over three times compared to the situation where the tube wall fouling does not exist.     

Thermal Performance of Coiled Square Tubes at Large Temperature Differences for Heat Exchanger Application

In many heat exchanger applications, working fluid inside the tubes is subjected to considerable temperature changes. Coiled tubes are used widely in heat exchanger applications due to the enhanced heat transfer rate caused by secondary flows. This study examines the thermal performance of three configurations of coiled tubes of square cross-section, namely, in-plane, helical, and conical coiled tubes, subjected to a large temperature difference between the fluid and the wall and compares it with that of a straight tube of identical cross-section area and length. The concept of figure of merit (FoM) is introduced to compare the heat transfer performance of the various configurations tested. The results indicate that FoM increases as the wall temperature is increased. In addition, the combination of temperature-induced buoyant flow and curvature-induced secondary flow significantly affects the flow behavior and heat transfer performance inside the tubes. The coil pitch in helical and conical tubes has an adverse effect on the heat transfer performance due to shift in vortices generation. The in-plane spiral tube operates at a higher wall temperature and lower Reynolds number, which gives rise to a higher FoM. The highest Nusselt number is obtained for the in-plane spiral tube at higher wall temperature and higher Reynolds number, which shows potential for practical applications.     

Air side heat transfer coefficients of discrete plate finned-tube heat exchangers with large fin pitch

The objective of this study is to investigate the heat transfer characteristics of discrete plate finned-tube heat exchangers with large fin pitches. Thirty-four heat exchangers were tested with variations of fin pitches, the number of tube rows, fin alignment, and vertical fin space. The j-factor of the discrete plate finned-tube exchanger was analyzed as a function of coil geometry and then compared with that of the continuous plate finned-tube heat exchanger. For fin pitches of 7.5–15 mm, the j-factors of the discrete plate finned-tube heat exchangers were 6.0–11.6% higher than those of the continuous plate finned-tube heat exchangers. Two separate correlations for the j-factor were developed for the inline and the staggered fin alignment in the discrete plate finned-tube heat exchangers to predict the measured data within a relative deviation of 2.9%.     

Boiling Heat Transfer and Pressure Drop of a Refrigerant R32 Flowing in a Small Horizontal Tube

In this study, experiments were performed to examine characteristics of flow boiling heat transfer and pressure drop of a low global warming potential refrigerant R32 flowing in a horizontal copper circular tube with 1.0 mm inside diameter for the development of a high-performance heat exchanger using small-diameter tubes or minichannels for air conditioning systems. Axially local heat transfer coefficients were measured in the range of mass fluxes from 30 to 400 kg/(m²·s), qualities from 0.05 to 1.0, and heat fluxes from 2 to 24 kW/m² at the saturation temperature of 10°C. Pressure drops were also measured in the rage of mass fluxes from 30 to 400 kg/(m²·s) and qualities from 0.05 to 0.9 at the saturation temperature of 10°C under adiabatic condition. In addition, two-phase flow patterns were observed through a sight glass fixed at the tube exit with a digital camera. The characteristics of boiling heat transfer and pressure drop were clarified based on the measurements and the comparison with data of R410A obtained previously. Also, measured heat transfer coefficients were compared with two existing correlations.     

An Experimental Study on the Air-Side Heat Transfer Coefficient and the Thermal Contact Conductance in Finned Tubes

The objective of this experimental study is to evaluate the heat transfer coefficient outside a tube with annular transverse fins, derived from strips of copper mechanically bound and coupled outside. Water is used as the heating medium, in turbulent conditions and flowing at different temperatures inside the tube. Petukhov's correlation has been selected to calculate the water heat transfer coefficient in the tube. The experimental data obtained are compared with a correlation from literature, and a similar trend is observed. A fitting of the data provides a correlation for the three tubes of different external diameter (30 mm, 22 mm, and 15.6 mm) that agrees very well with the experimental values. The thermal contact conductance is identified as the main reason for the difference between data and the original Briggs and Young correlation. An estimation of the contact conductance between fins and tubes provides values between 3500 and 11000 W/m²-K, slightly increasing with the air Reynolds number (based on the external diameter of the tube), whose range is 2000 to 8000. The thermal contact resistance is estimated and its importance is confirmed, contributing 30 to 50% to the total air-side thermal resistance in the tubes used in the experiments.     

Performances Cost Effectiveness,and Water-Side Fouling Considerations of Enhanced Tube Heat Exchangers for Boiling Service with Tube-Side Water Flow

A case study method is employed to calculate the performance benefits and cost effectiveness of enhanced tubes for an application involving seawater on the tube side and ammonia boiling on the shell side. A porous boiling surface is used on the ammonia side, and eight basic geometries are evaluated for the water side. The analysis method selects the “optimum” enhancement dimensions for each geometry type. The optimum is de fined as the minimum material requirement for fixed heat duty and water-side pumping power. Heat exchanger size reductions of 55–67% are possible, depending on the tube-side enhancement type. The cost effectiveness of the enhanced tube designs are calculated relative to a plain tube exchanger for aluminum, copper, and titanium materials. Heat exchanger tubing cost reductions in the range of 10% are predicted for the titanium and copper tubes. Doubly enhanced aluminum tubes do not appear to provide heat exchanger cost reduction. A better application for enhanced tubes may involve increasing the UA value to provide a reduced LMTD. The reduced LMTD can be used to increase process thermodynamic efficiency, and thus offset the greater heat exchanger cost. The results of this study must be considered tentative until the water-side fouling characteristics of enhanced tubes are established.     

Boiling Heat Transfer and Pressure Drop of a Refrigerant Flowing Vertically Upward in Small Rectangular and Triangular Tubes

In the present study, experiments were performed to examine the characteristics of flow boiling heat transfer and pressure drop of a refrigerant R410A flowing vertically upward in small copper rectangular and triangular tubes with hydraulic diameters of 1.04 and 0.88 mm, respectively, for the development of a high-performance heat exchanger using small tubes or minichannels for air-conditioning systems. Their characteristics were clarified by comparing the previous experimental data of the small circular tube (1.00 mm internal diameter). In the rectangular and triangular tubes, the pressure drop was found to be slightly lower and the heat transfer was much better than in the circular tube.     

Experimental Investigation on Flow Boiling Heat Transfer and Pressure Drop of HFC-134a inside a Vertical Helically Coiled Tube

An investigation on flow boiling heat transfer and pressure drop of HFC-134a inside a vertical helically coiled concentric tube-in-tube heat exchanger has been experimentally carried out. The test section is a six-turn helically coiled tube with 5.786-m length, in which refrigerant HFC-134a flowing inside the inner tube is heated by the water flowing in the annulus. The diameter and the pitch of the coil are 305 mm and 45 mm, respectively. The outer diameter of the inner tube and its thickness are respectively 9.52 and 0.62 mm. The inner diameter of the outer tube is 29 mm. The average vapor qualities in test section were varied from 0.1 to 0.8. The tests were conducted with three different mass velocities of 112, 132, and 152 kg/m²-s. Analysis of obtained data showed that increasing of both the vapor qualities and the mass fluxes leads to higher heat transfer coefficients and pressure drops. Also, it was observed that the heat transfer coefficient is enhanced and also the pressure drop is increased when a helically coiled tube is used instead of a straight tube. Based on the present experimental results, a correlation was developed to predict the flow boiling heat transfer coefficient in vertical helically coiled tubes.     

A Large Eddy Simulation of Plate-Fin and Tube Heat Exchangers with Small Diameter Tubes

Plate-fin and tube heat exchangers are extensively studied both experimentally and numerically. However, data on the fluid flow and heat transfer in the exchanger passage with small diameter tubes have not been accumulated enough. With a large eddy simulation technique (LES), this study performs a detailed investigation of the fluid flow and heat transfer in a plate-and-tube channel with tubes of diameters as small as 5.2 mm. The conservation equations for mass, heat, and momentum were solved by the proposed LES model. It was found that the LES model is appropriate to predict the fluid flow and heat transfer. Compared to heat exchangers of larges tubes, the heat exchangers exhibit much higher heat transfer coefficients with small tubes. The fin efficiencies are improved with small tubes.     

Finned-Tube Heat Exchanger Optimization Methodology

Heat exchangers play a dominant role in the performance of most energy systems; however, optimization of these components is a complex task due to the coupled nature of the design parameters involved. Anytime the heat transfer coefficient is increased in these components, there is a corresponding increase in frictional pressure drop; therefore, a delicate balance is required between these two effects in heat exchanger optimization.

The finned-tube condenser heat exchangers used in residential air conditioning systems are examined in this study. Due to the intricate geometry of the finned-tube heat exchanger, there are no analytical optimization schemes available to optimize their design, while experimental trial and error is far too time consuming, considering the ten different design parameters that can be varied for optimization.

This study develops a system model using available analytical and empirical correlations for the entire air conditioning cycle with great detail in the condenser component. An optimization algorithm then uses this model to find an optimum design for ten condenser design parameters using various constraints with a system COP figure of merit. The design optimization methodology is fully developed and presented in the paper so that it can be applied to other energy systems' heat exchanger optimization opportunities.

The optimum condenser design was found to give the same performance as a coil optimized through a manual search costing 23% more. It is also shown that the optimum design is consistent with minimum entropy generation for the total system.     

EHD Augmented Convective Boiling: Flow Regimes and Enhanced Heat Transfer

This work investigates the influence of electrohydrodynamics (EHD) on the flow and heat transfer during convective boiling of HFE7000. A unique tube-and-shell heat exchanger has been constructed with heated water flowing on the shell side and a saturated mixture of refrigerant flowing within the tube side. The heat exchanger is novel in that it allows full visual access to the flow in the inner tube while being both thermally and electrically conductive. This permits observation of the two-phase flow regimes, which is not possible with metallic test sections. In this work the influence of EHD on the flow regimes and subsequent overall heat transfer is investigated for fixed inlet refrigerant mass flux of 100 kg/m²-s, inlet quality of 3%, and wall superheat of approximately 11.5°C. For these conditions the applied voltage across a concentric inner electrode and the outer wall of the tube was varied between 0 kV and 10 kV at 60 Hz AC. In particular, this work focuses on quantifying the level of overall enhancement that is achievable with EHD for this heat exchanger. This is done in the context of the additional heat extracted by the working fluid in the heat exchanger compared with the field-free case and the additional power penalties required to do so. Heat transfer enhancements of up to 1.8 -fold were realized in this heat exchanger. Even so, there were hydraulic power increases as well as electrical power required to achieve the heat transfer enhancement. It was found that the electrical power was the dominant penalty and that an overall enhancement of 40 times more heat power extracted than input required was achieved. Finally, a proportional–integral–derivative (PID) control system has been utilized in conjunction with a high-voltage amplifier in order to accurately control the heat transfer rate of the heat exchanger. To our knowledge this is the first solid-state control system of this type for a two-phase heat exchanger.     

Fluid Flow and Heat Transfer in Plate-Fin and Tube Heat Exchangers in a Transitional Flow Regime

The unsteady behaviors of fluid flow and heat transfer in plain plate-fin and tube heat exchangers with a wide range of fin spacings from 2.06 mm to 16.48 mm and tube diameter 8.28 mm are studied by a large eddy simulation technique (LES). Velocity fluctuations and vortex sheddings induced by the tubes in the channel are modeled. The results found that the flow in passages of large spacings is quite different from that of small spacings. The flow is co-determined by two effects: the duct effect and the tube bank effect. The tube bank effect is more dominant with increasing fin spacings.     

Experimental Determination of the Boiling Heat Transfer Coefficients of R-134a and R-417A on a Smooth Copper Tube

Flooded evaporators are widely used as compact cooling units to cool liquids. They consist of a shell-and-tube heat exchanger, with the fluid to cool flowing inside the tubes of the bundle and a refrigerant that evaporates over those tubes. Pool boiling on the external surface of the tubes is a very complex process, and therefore the boiling heat transfer coefficients (HTCs) should be determined experimentally. Copper and copper alloys tubes are commonly employed in such heat exchangers, due to their high thermal conductivity and relative low cost. On the other hand, refrigeration and air conditioning sectors are undergoing significant changes caused mainly by the necessity of replacing existing refrigerants with more environmentally friendly ones. This paper reports the experimental determination of the pool boiling HTCs of R-134a and R-417A blend on a smooth copper tube of 18.87 mm diameter, at two saturation temperatures of 10°C and 7°C. Although smooth tubes are not commonly used in shell-and-tube evaporators nowadays, it is a first approach to pool boiling of drop-in refrigerants. The experimental setup and data acquisition are described, the experimental procedure is explained, the data reduction methodology is detailed, and the results are presented and discussed.     

扭曲椭圆管换热器壳程强化传热的数值研究

基于扭曲椭圆管的换热器是一种新型的新风系统换热器,针对扭曲椭圆管及其应用特点,设计了两种不同结构参数的新风系统换热器。应用FLUENT软件,在夏季工况下对两种不同结构参数的新风系统换热器壳程进行模拟分析,并通过与实验数据的对比,验证计算模型的可靠性。结果显示在相同体积流量下,随着壳程开孔面积的增大,对流换热系数h不断减小,压降Δp不断减小,综合性能系数h/Δp^1/3变化不明显;随着螺距的减小,对流换热系数h不断增大,压降Δp不断增大,综合性能系数h/Δp^1/3也不断增大;流场分析显示,扭曲椭圆管换热器壳程流道内,呈现出明显沿着扭曲椭圆管壁面的螺旋流,使得空气在流道内充分扰动,增强换热效果。     

Prediction and Measurement of Apparent Heat Transfer Coefficient by Condensation in Finned-Tube Heat Exchangers

The apparent heat transfer coefficient by condensation in finned-tube heat exchangers is determined experimentally and numerically in this paper. The film method is used to predict the partial or total condensation of the water vapor contained in the humid air over the smooth or finned tube-heat recuperators. Based on this method, a computer code is developed here. The mathematical formulation is validated by our experimental results, using tube bundles in staggered and aligned arrangements. The determination of the fin portion, which functions in wet regime, is carried out by the prediction of thermal field over a circular fin. The condensation of the water vapor contained in the humid air is done preferentially with the last rows of the heat exchanger. The heat flux is predicted in a range of 20% and 5% in wet and dry regimes, respectively. The apparent heat transfer coefficient by condensation can exceed 10 times the value of the air-side heat transfer coefficient in dry regime.