Mantle heat exchangers for horizontal tank thermosyphon solar water heaters

This paper describes the characteristics of horizontal mantle heat exchangers for application in thermosyphon solar water heaters. A new correlation for heat transfer in horizontal mantle heat exchangers with bottom entry and exit ports was used to predict the overall heat transfer and stratification conditions in horizontal tanks with mantle heat exchangers. The model of a mantle heat exchanger tank was combined with the thermosyphon solar collector loop model in TRNSYS to develop a model of a thermosyphon solar water heater with collector loop heat exchanger. Predictions of stratification conditions in a horizontal mantle tank are compared with transient charging tests in a laboratory test rig. Predictions of daily energy gain in solar preheaters and in systems with in-tank auxiliary boosters are compared with extensive outdoor measurements and the model is found to give reliable results for both daily and long-term performance analysis.     

Heat transfer correlations for vertical mantle heat exchangers

This paper investigates heat transfer in vertical mantle heat exchangers for application in low flow solar domestic hot water systems. Two new heat transfer correlations for vertical mantle heat exchangers with top entry port and bottom exit ports are developed. The correlations are based on computational fluid dynamic modelling of whole vertical mantle tanks. The correlations are combined with a heat storage model in a simulation program that predicts the yearly thermal performance of low flow solar domestic hot water systems based on mantle tanks. The model predictions of energy gains and temperatures are compared with outdoor measurements and the model is found to give reliable results.     

Some aspects concerning modelling the flow and heat transfer in horizontal mantle heat exchangers in solar water heaters

An experimental and numerical investigation has been undertaken to study the heat transfer process in horizontal mantle heat exchangers used in solar water heaters. A rectangular cavity has been used as a simplified geometry. With the aid of particle image velocimetry (PIV) the flow field in the centre‐plane of the rectangular cavity has been visualized. Three‐dimensional flow simulations were performed using a commercial CFD package. The impinging jet formed by the inlet flow directed towards the opposite wall was found to produce localised turbulence in the cavity, with an inlet Reynolds number as low as 360. This turbulence was found to effect the flow field and heat transfer in the cavity when the inlet Reynolds number was above 1200. It is shown that, with the boundary conditions used in this study, most of the heat transferred was in the bottom half of the cavity. This is not the ideal situation for optimization of solar water heating systems. Copyright © 1999 John Wiley & Sons, Ltd.     

Performance of solar water heaters with narrow mantle heat exchangers

This paper evaluates the performance of narrow-gap vertical mantle heat exchangers with a two-pass arrangement for use in pumped-circulation solar water heaters. Both measured mantle side and tank side heat transfer correlations have been developed and implemented in a TRNSYS model of a complete solar water heater incorporating this type of heat exchanger. Predictions of the annual solar contribution for mantle-tank systems are compared to direct-coupled systems. The direct-coupled systems are found to provide slightly higher annual energy saving than mantle-tank systems for standard domestic hot water demand in Australia. However, the reduction in performance is outweighed by the benefit of freeze protection provided by incorporating a collector loop heat exchanger in the system.     

Analysis of the flow structure and heat transfer in a vertical mantle heat exchanger

The flow structure inside the inner tank and inside the mantle of a vertical mantle heat exchanger was investigated using a full-scale tank designed to facilitate flow visualisation. The flow structure and velocities in the inner tank and in the mantle were measured using a Particle Image Velocimetry (PIV) system. A Computational Fluid Dynamics (CFD) model of the vertical mantle heat exchanger was also developed for a detailed evaluation of the heat flux at the mantle wall and at the tank wall. The flow structure was evaluated for both high and low temperature incoming flows and for both initially mixed and initially stratified inner tank and mantle. The analysis of the heat transfer showed that the flow in the mantle near the inlet is mixed convection flow and that the heat transfer is dependent on the mantle inlet temperature relative to the core tank temperature at the mantle level.     

Detailed Loop Model (DLM) analysis of liquid solar thermosiphons with heat exchangers

An analytical Detailed Loop Model (DLM) has been developed to analyze the performance of solar thermosiphon water heaters with heat exchangers in storage tanks. The model has been used to study the performance of thermosiphons as a function of heat exchanger characteristics, heat transfer fluids, flow resistances, tank stratification, and tank elevation relative to the collector. The results indicate that good performance can be attained with these systems compared to thermosiphons without heat exchangers.     

Discharge of a thermal storage tank using an immersed heat exchanger with an annular baffle

A number of solar domestic hot water systems and many combined space and water heating systems have heat exchangers placed directly in the storage fluid to charge and/or discharge the tank. Operation of the heat exchanger produces a buoyancy-driven flow within the storage fluid. With a view toward controlling the flow field to increase heat transfer, a cylindrical baffle is inserted in a 350 l cylindrical storage tank. The baffle creates a 40 mm annular gap adjacent to the tank wall. A 10 m-long, 0.3 m² copper coil heat exchanger is placed in the gap. The effects of the baffle on the transient heat transfer, delivered water temperature, heat exchanger effectiveness, and temperature distribution within the storage fluid are presented during discharge of initially thermally stratified and fully mixed storage tanks. The baffle increases the storage side convective heat transfer to the heat exchanger by 20%. This increase is attributed to higher storage fluid velocities across the heat exchanger.     

强制循环式太阳热水系统动态特性分析

基于集热器，水箱及换热器等部件热,地晴天无负荷条件下运行的强制循环式太阳能热水系统进行数值模拟，分析了贮热水箱内温度分层，水量，高径比和水流率等对瞬时集热效率和系统日效率的影响。特别探讨了带热交换器的复合回路系统在两种介质热容流率比值改变时，系统热性能变化规律。对设计和控制运行强制循环式太阳热水系统提出了一些建议。     

Experimental and numerical study of thermal stratification in a mantle tank of a solar domestic hot water system

The simulation and the optimisation of the mantle tank of solar domestic hot water systems needs dynamic simulation over long periods of time (e.g. 1 year). A model for such a mantle tank was developed by using the zonal approach. The dimensions of the zones are determined based on physical considerations. A mixing coefficient is identified to model the water flow in the mantle heat exchanger. Comparisons of the results of temperatures distribution of the model and of experiments show a difference <7% for three positions of the inlet water flow in the mantle heat exchanger.     

Characteristics Of Vertical Mantle Heat Exchangers For Solar Water Heaters

The flow structure in vertical mantle heat exchangers was investigated using a full-scale tank designed to facilitate flow visualisation. The flow structure and velocities in the mantle were measured using a particle Image Velocimetry (PIV) system. A CFD simulation model of vertical mantle heat exchangers was also developed for detailed evaluation of the heat flux distribution over the mantle surface. Both the experimental and simulation results indicate that distribution of the flow around the mantle gap is governed by buoyancy driven recirculation in the mantle. The operation of the mantle was evaluated for both high and low temperature input flows.     

System modeling and operation characteristics of thermosyphon solar water heaters

An efficient numerical simulation model for thermosyphon solar water heaters has been developed and compared with test data from two locations. The model was used to study the characteristics of vertical and horizontal tank thermosyphon systems. The results indicate that thermosyphon systems have optimum performance when the daily collector volume flow is approximately equal to the daily load volume. Heat conduction in one tank horizontal system was found to significantly reduce solar contribution.     

Optimum mass flow rate of water in a flat plate solar collector coupled with a storage tank and an organic Rankine cycle power loop

The optimum flat plate solar collector operation for maximizing solar organic Rankine cycle work output is investigated. A mathematical model for the flat plate solar collector water system integrated with a solar storage tank and an organic Rankine cycle loop is developed. The mass flow rate of water is searched for to obtain the maximum net work output from the integrated system during 1 yr of operation. Curve fittings of thermodynamic properties of some chosen organic fluids (R-11, R-21 and R-113) are derived for computer use. The Hooke and Jeeves optimization technique is used to determine the optimum mass flow rate of water in the collector for different values of collector inner tube diameters and boiler heat transfer coefficient-area product.     

Comparison of solar heat exchangers

This paper summarizes and compares the theoretical heat transfer characteristics of solar heat exchangers. Comparisons are made with a number of heat exchanger targets used in solar energy applications. The efficiencies of flat-plate, circular flat-plate, cylindrical, cylindrical annulus, spherical annulus and elliptic cylindrical heat exchangers are presented for comparison. Simplified lumped analysis expressions are derived for these heat exchangers, and the results compare very well with the more complicated distributive analysis results over a range of Nusselt numbers frequently encountered in solar energy applications. The influence of thermal and velocity profiles is also discussed. From this study it appears that the annular geometry yields higher efficiencies especially at large Nusselt numbers. A secondary aim of this paper is to provide a summary of the heat transfer characteristics of heat exchangers with different geometries in sufficient detail that would allow a designer of solar energy equipment to quickly make calculations for a particular application.     

Heat transfer and two-phase flow during convective boiling in a partially-heated cross-ribbed channel

Measured local heat transfer data and visual observations of the two-phase flow behavior are reported for convective boiling of saturated liquids in a cross-ribbed channel similar to geometries used in formed-plate compact heat exchangers. Experiments in this study were conducted using a special test section which permitted direct visual observation of the boiling process while simultaneously measuring the local heat transfer coefficient at several locations along the channel. One wall of the channel was heated while the opposite and lateral walls were adiabatic. Measured local heat transfer coefficients on the heated portion of the channel wall were obtained for convective boiling of methanol and n-butanol at atmospheric pressure with the channel oriented vertically and in horizontal positions with top heating, side heating and bottom heating of the channel. Vertical flows were observed to be in the churn or annular flow regimes over most of the channel length whereas the horizontal flows were either in the wavy or annular flow regime over most of the channel. Visual observations also indicated that virtually no nucleate boiling was present when the flow was in one of these three regimes. For the same coolant and flow conditions, at moderate to high qualities, the measured convective boiling heat transfer coefficients for the vertical and horizontal orientations were usually found to differ by only a small amount. However, for some orientations, partial dryout of the heated wall of the channel was sometimes observed to reduce the heat transfer coefficient. A method of correlating the heat transfer data for annular film-flow boiling in cross-ribbed channel geometries is also described.     

Experiment of flow regime map and local condensing heat transfer coefficients inside three dimensional inner microfin tubes

INTRODUCTIONSince1980s,thestudyofthein-tubecondensingheattransferenhancementhasbecomeimportant.Thetwodimensionalinnerfintubes,twodimensionalinnermicrofintubes(inthefollowingsimplycalled2Dtubes)andinserterswerepaidattentiontoforthein-tubecondensinghea...     

On the performance of a vertical helical coil heat exchanger. Numerical model and experimental validation

A numerical model was developed in order to predict the heat transfer process and pressure drop in a vertical helical coil heat exchanger (HCHE) located inside a fluid storage tank in which water is used as inner and outer fluid. Natural convection was considered as boundary condition for the HCHE outer surface. The model was validated with experimental data obtained from an own facility with two HCHEs tested under several operating conditions. The model developed was used to evaluate the main HCHE representative geometrical parameter's influence on the overall heat transfer coefficient and pressure drop. The results show that by increasing the tube diameter causes an increase of the Nusselt number and a larger heat transfer rate to pressure drop ratio is obtained.     

A void fraction model for annular flow in horizontal tubes

An important feature of detailed system simulation models for unitary air conditioners is the calculation of charge inventory. Void fraction determination in the two-phase regions of the heat exchangers is the primary challenge associated with charge inventory calculations. Annular flow is one of the predominant flow regimes encountered in horizontal heat exchangers. Analytical annular flow models typically fail to accurately represent void fraction. Thus, many of the available void fraction models are empirically based. To improve the prediction capabilities of void fraction models, a mechanistic void fraction model has been developed for annular flow in horizontal tubes. The present model considers the effect of momentum eddy diffusivity damping at the liquid-vapor interface. Two approaches are presented for determining the wall shear stress. The modeling results are compared to predictions from various void fraction models found in the literature. The present model is found to work well at moderate mass fluxes.     

SIMULATION OF SWIRLING TURBULENT FLOWS AND HEAT TRANSFER IN AN ANNULAR DUCT

The article presents a numerical simulation of swirling turbulent flows and heat transfer in an annular duct. The time-averaged governing equations are solved, which are closed by a new algebraic Reynolds stress model (ASM). The simulation is performed under different flow conditions. The calculated results of gas axial and tangential velocities, turbulent kinetic energy, temperature, and local heat transfer coefficients on the inner and outer walls of the annulus are provided. They illustrate the effect of swirl number, inlet axial velocity, and ratio of inner to outer radius on the mean flow and turbulence properties, as well as on enhancing heat transfer in the annular duct.     

Heat Transfer and Pressure Drop Characteristics of Smooth Horizontal Tubes in the Transitional Flow Regime

The operating conditions of many heat exchangers are in, or close to, the transitional flow regime. However, in this regime, not a lot of design information is available and some design books even recommend to not design heat exchangers to operate in the transitional flow regime. Furthermore, it is known that the type of inlet of heat exchangers influences the transition characteristics. It was therefore the purpose of this study to measure heat transfer and pressure drop characteristics in smooth horizontal tubes using different types of inlets. The types of inlets were hydrodynamically fully developed, square-edged, re-entrant, and bellmouth. Experiments were conducted on a 14.48-mm inner diameter horizontal tube in which the water was cooled. Reynolds numbers ranged between 1000 and 20,000 and Grashof numbers were on the order of 10⁵. It was found that for adiabatic flow the square-edged inlet delayed transition to Reynolds numbers of around 2600, while the bellmouth inlet delayed it to about 7000. However, for diabatic flow, the transition was independent of the type of inlet. Laminar friction factors were much higher than their theoretically predicted values due to the secondary flows increasing the amount of mixing in the tube. Heat transfer measurements showed that transition with water was totally independent of the type of inlet used.     

垂直环管内旋流换热与流动的数值模拟研究

基于垂直环管内旋流对流动边界层的扰动机理,采用数值模拟的方法研究了叶片角度、雷诺数以及进口水温对管内换热以及流动特性的影响,揭示了重力对环管内旋流流动的内在影响机制。结果表明：与水平环管相比,垂直环管的综合换热性能变化平缓,主要受到重力对压降的影响;与雷诺数相比,叶片角度对流场以及温度场的影响最显著;在雷诺数小于15 000,叶片角度为30°时管内的换热性能最佳。