Understanding Mantle Heat Exchangers Used in Solar Water Heaters

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1 Understanding Mantle Heat Exchangers Used in Solar Water Heaters Gary Rosengarten, Graham L. Morrison and Masud Behnia School of Mechanical and Manufacturing Engineering The University of New South Wales Sydney NSW, 2052 AUSTRALIA Telephone: +61 (0) Facsimile: +61 (0) Abstract Most of the 30,000 or so solar water heaters manufactured each year in Australia are low-flow thermosyphon systems that use a horizontal mantle heat exchanger to transfer heat from the collector fluid to the storage tank. An important aspect of the mantle heat exchanger is the ability to transfer heat to the correct regions of the tank in order to avoid thermal de-stratification. This paper gives initial results that have used a geometrical simplification of the mantle, a rectangular cavity, to examine the flow field with controlled tank wall boundary conditions. It is shown that improvements can be made to increase the heat exchanger s effectiveness. 1 INTRODUCTION It would be convenient if there was one type of domestic solar hot water heating system that was globally recognised as the best. This is not the case and, in fact, there are a plethora of designs used around the world that differ, generally, in relation to the environment that they are designed for (Morrison, 1995). In parts of Australia, for example, despite the fact that there is plenty of solar radiation, the night temperature can often be low enough to freeze water. This is especially so for water in solar collectors, as their major mode of heat loss is by radiation to the sky that may be much colder than the air temperature. To ensure no freezing and minimal fouling occurs in the collector loop, a water/glycol mixture is used, which is thermally connected to the hot water storage tank by the use of a mantle heat exchanger MHE (Figure 1). The MHE consists of an annular jacket, around the storage tank, in which the hot collector-loop-fluid flows, without an electric pump, due to thermosyphoning. This utilises the density change with temperature, which is a function of vertical position in the collector loop. Thermosyphon based solar water heaters are superior to those using mechanical pumps in terms of their lower capital cost, maintenance and auxiliary energy usage. Storage tank Potable water To collector From collector Mantle heat exchanger Figure 1: Schematic of mantle heat exchanger used in SDHW systems (width of mantle exaggerated)

2 The are two other types of heat exchangers that have been studied for use in thermosyphon solar collectors. The first type, which uses a heat exchanger inside the storage tank, was initially studied by Mertol et al (1981). They numerically modelled a thermosyphon solar water heater, with a heat exchanger system that consisted of copper tubes inside the storage tank. They assumed a fixed value for the heat transfer coefficient, h, of 170W/m. 2 K. Webster et al (1987) extended this work to ascertain the effect of thermosyphon performance using a heat exchanger in the tank. They showed that if a series of copper tubes at the bottom of the tank were used, little stratification occurred relative to the direct heating case (hot water flowing directly into the tank). The second type utilises a shell and tube heat exchanger external to the tank. This configuration uses a thermosyphon loop on the tank side for which cold tank water from the bottom of the tank enters the shell and is returned heated into the top of the tank. The collector fluid is driven by a pump, and flows in the tubes in a direction counter to the tank flow. These types of heat exchangers were first investigated by Parent et al 1990 using a forced convection model. They showed that the heat exchanger effectiveness ranged from 40-90% but they also used a constant h value through the whole heat exchanger. For a summary of other work done on external shell and tube see Dahl and Davidson (1997). The mantle heat exchanger is possibly superior to the previous two types for the following reasons. Simplicity of design and smaller space requirements, by combining the hot water tank and heat exchanger into one unit. Larger heat exchange surface area. A mantle heat exchanger surface area would be at least two and a half times larger than the eight immersed copper tubes used in the study by Webster et al (1987). Higher efficiency. Comparing three low flow solar water heating systems with heat exchangers it was found that the vertical mantle type outperformed the immersed coil and external shell and tube type (Furbo, 1993). The use of mantle heat exchangers can be divided into two categories. Denmark is a large user of vertical mantle heat exchangers for which the storage tank is vertical, with the mantle only covering the bottom half of the tank. The collector fluid is pumped through the annular gap, entering at the top and leaving at the bottom. Shah and Furbo (1997) have found that heat transfer from the collector fluid to the inner tank is dominated by natural convection and have found a correlation between the Nusselt number and Rayleigh number as a function of height. In Australian systems the mantle is horizontal and the flow relies solely on thermosyphoning, thus leading to quite different characteristics. Nasr et al (1996) and Morrison et al (1997) have shown that the flow in a horizontal mantle heat exchanger is in the mixed convection regime. Due to the effect of buoyancy forces, unwanted recirculation zones develop. These have the effect of transferring heat from the top to the bottom of the tank thus decreasing tank stratification. 1.1 An annulus becomes a rectangle The annular shape of the mantle is an awkward shape to use for experimental flow visualisation due to curvature of the flow plane (ie. it is difficult to view the flow at the bottom inlet and the top simultaneously). However, because of the large ratio of the annulargap width to the mean diameter (~60) of the annulus, it is possible to approximate the annular flow with flow in a vertical rectangular cavity. The conceptual steps are shown in Figure 2. Due to symmetry only half the mantle needs to be modelled (a-b) and because of the small gap width this annular shape can be opened up to form a rectangle (b-c). It has been shown numerically that, for the type of flows that occur in mantle heat exchangers, the flow and heat transfer characteristics are very similar for both the annular and rectangular shapes (Nasr et al (1997)) a b c Figure 2: Turning an annulus into a rectangle. Figure Figure 3: 2: Schematic of of mantle mantle heat heat exchanger used used in in SDHW SDHW systems systems (width (width of of mantle mantle exaggerated). 2 Proceedings of Solar 97 - Australian and New Zealand Solar Energy Society Paper 101

3 The advantage of using a rectangular cavity is that all the flow is essentially in one plane so that flow visualisation using dye injection becomes much easier and other very useful flow diagnostic tools like particle imaging velocimetry (PIV) become possible. The mantle heat exchanger needs extra care to be analysed, as it s heat-exchanging properties are strong functions of horizontal and vertical directions in the heat-exchanging plane. It is well known that in order to improve the efficiency of SDHW systems, the hot water in the tank should be as stratified as possible, so that most of the heat is in the top of the tank where load in drawn. For this reason it is not sufficient to define a heat exchanger, that is used to transfer heat into a thermal storage medium, only in terms of the amount of heat supplied. This paper discusses the first step in fully understanding and defining mantle heat exchangers by simplifying the geometry. 2 THE MODELS 2.1 Experimental A rectangular cavity was designed to be able to have precisely controlled boundary conditions. It consists of a 5mm thick copper plate, a 10mm thick perspex spacer and a 10mm thick perspex front (Figure 3). The internal dimensions of the flow channel are 480x230x10mm. The temperature of the copper plate is controlled by using two water channels attached at the back. The water in each channel is pumped from two temperature controlled water baths with a stability of better than ±0.2 o C. Twenty-seven type T thermocouples are embedded in the copper block with the tips about 1mm from the front surface, to measure the temperature distribution on the front face. Another two thermocouples are attached to the inlet and outlet pipes. The thermocouples are connected to a Yokogawa hybrid recorder that scans through the thermocouple readings and sends temperature values to a PC via an RS232 connection. The type T thermocouples have a calibration uncertainty of ±0.2 o C. Thermocouples Temperature recorder Temperature controlled water baths (with pumps) PC Dye injection tubes Temperature controlled water source Cu plate Water channel Perspex front Perspex spacer Figure 3: Experimental arrangement for rectangular cavity flow visualisation. The inlet water is provided by a pumped, circulated,70-litre constant temperature water source. Flow rates are varied using a control valve and measured using a calibrated rotameter. Buoyancy neutral dye (methyl blue) is released through 0.8mm (outer diameter) stainless steal hypodermic tubing into the cavity for flow visualisation. The photos for flow visualisation are taken using a CCD camera. Proceedings of Solar 97 - Australian and New Zealand Solar Energy Society Paper 101 3

4 2.2 Numerical Model The full 3D Navier-Stokes momentum equations and static enthalpy transport equations were solved assuming laminar, steady flow, using the commercial CFD software Fluent. A computational grid, which was contracted around areas of high gradients (outlet and inlet), of about grid points was used. The Bousinesq model was used for calculating buoyancy forces. Power-Law interpolation was used for convective differencing. The Line Gauss-Sidel (LGS) iterative scheme was implemented for the velocity components and a multigrid technique used for the enthalpy and pressure equations. 3 RESULTS AND DISCUSSION Figure 4 shows a comparison between the dye streaklines and the numerically determined particle streaks (in the mid plane of the cavity), under isothermal conditions, with both the inlet water and the inner wall at ambient temperature (~20 o C). The inlet configuration used in typical Australian mantle solar water heaters is reproduced in this set up. The inlet is perpendicular to the plane of inner wall, thus as the water enters the rectangular cavity (and the annulus) it impacts the inner tank wall in a manner that can be described as jet impingement. The experimental and numerical streaklines shown in Figure 4 are in reasonable agreement for cases a and b. For case c, which has the largest inlet flow rate, the mean flow represented by the experimental streaklines is similar to numerical results, but there is an unsteadiness that is not accounted for in the numerical simulation. The unsteadiness is localised around the inlet port and is due to the instability caused by the jet impingement. Buenconsejo (1994) has observed this phenomenon, although to a lesser extent, in a perspex model of the annular heat exchanger. The impingement was probably not as severe as in the rectangular case because the inlet diameter was larger and the impingement surface was curved. a b c Inlet The effect of jet impingement is a function of inlet velocity and temperature gradients. It was found that the effect was amplified when temperature gradients were present so that the flow rapidly became turbulent around the inlet. This is illustrated in Figure 5a, which represents a stratified tank. The inner wall temperature Figure 4 Streak lines for isothermal conditions (20 o C) and inlet flow rates of a) 135ml/min, b) 200ml/min and c) 700ml/min. The white lines are the numerical results. varied linearly from 20 to 45 o C, the inlet temperature was 45 o C and the flow rate was 200ml/min. There is a near stagnant zone close to the top, some faster moving turbulent-like flow in the middle and some slow moving reversed flow at the bottom. It was not possible to view streaklines in the region above the inlet, as the flow was turbulent, causing rapid dispersion of the dye. The laminar numerical model that used an inlet temperature of 57 o C, a stratified tank wall from 27 to 57 o C, and an inlet flow 200ml/min (Figure 5b) shows velocity vectors that are consistent with the 4 Proceedings of Solar 97 - Australian and New Zealand Solar Energy Society Paper 101

5 experimentally observed flow field at the mid plane of the cavity. There is fast flow up the side wall and a recirculation zone to the left of the inlet. Figure 5c shows the heat flux on the inner wall (into the storage tank). The large values near the inlet are expected to increase when turbulence is taken into account. The modelling exercise leads to the ability to be able to characterise this heat exchanger in terms of how much, and where, heat is transferred. The average heat flux as a function of height is given in Figure 6. It was calculated by integrating the heat flux horizontally then dividing by the total horizontal length. Two other cases are compared, each with same inlet Reynolds number (905), based on the inlet diameter, as the rectangular case. They are both annular mantles with a 5mm gap width, 280mm inner diameter and 915mm long, and have the same stratification as the rectangular case (Morrison et al, 1997). The ideal heat transfer situation would be represented on Figure 6 by a curve that resembles a step input, with minimal heat being transferred at the bottom part of the tank and most being transferred at the top. This would be more closely represented if the inlet was at the top of the mantle, but this design has been avoided to minimise reverse circulation at night. At the moment it is a quite clear some of the fluid that is penetrating to the top of the tank has been sufficiently cooled to take heat out of the top. This combined with most of the heat being transferred into the bottom half of the tank means that the horizontal mantle heat exchanger has much room for improvement. The next step in this exercise is to find a parameter that represents the effect that the heat flux distribution has on the efficiency of hot b water storage. 4 CONCLUSION A rectangular cavity, that is a geometrical approximation of an annular cavity, has been used to model the flow and heat transfer characteristics of a mantle heat exchanger used in solar water heaters. Experimental results indicate that inlet conditions and wall boundary conditions are very important in terms of not only the mean flow, but in producing turbulence as well. It is shown that when the inner tank is stratified, and heated collector fluid flows through the heat exchanger, the heat transfer characteristics would tend to destratify the inner tank water. This unfavourable behaviour could be changed by repositioning the inlet or by inserting flow guides. c Figure 5: Dye streaklines (a) and velocity vectors (b) in the midplane of the cavity and heat flux (c) (in kw/m 2 ) on the back wall into the tank for a stratified inner tank (27-57 o C), Re=905 and Tin=57 o C. Proceedings of Solar 97 - Australian and New Zealand Solar Energy Society Paper 101 5

6 Normalised distance along tank wall (from bottom) Annular mantle 47 o C inlet temperature Annular mantle 57 o C inlet temperature Rectangular cavity 57 o C inlet temperature Mean heat flux into tank (W/m 2 ) Figure 6: Horizontally averaged heat flux into storage tank as a function of height for rectangular cavity and vertical distance along tank wall for the mantle. 5 REFERENCES Buenconsejo N.S.Jr. (1994) Experimental study of annular free convection heat exchangers: the flow characteristics in the annular gap. School of Mechanical and Manufacturing Engineering, The University of New South Wales, Australia Dahl S.D. and Davidson J. H. (1997). Performance and modelling of thermosyphon heat exchangers for solar water heaters, J. Solar Engineering, 119, Furbo S. (1993) Optimum design of small DHW low flow solar systems. ISES Solar World Congress, 1993, Budapest, Hungary. Mertol A. Place W. Webster T. and Greif R.(1981) Detailed loop model analysis of liquid solar thermosyphons with heat exchangers, Solar Energy, 27, Morrison G. L. Nasr A. Behnia M. and Rosengarten G. (1997) Performance of horizontal mantle heat exchangers in solar water heating systems, Paper presented at ISES 1997 World Congress, Taejon, Korea Morrison G. L.(1995) Design and rating of solar water heater storage tanks. Workshop, International Energy Agency, Solar Heating and Cooling Program, IEASHC/WS/2-95 Nasr A. Morrison G. L. and Behnia M. (1997). Computational study of flow and heat transfer characteristics of Annular and Vertical cavities. CASCM 97, Procs of 2 nd Sypm on Comp. Mech. Parent M. G. Van Der Meer H. and Hollands K. G. T. (1990). Natural convection heat exchangers in solar water heating systems: theory and experiment. Solar Energy, 38, Shah L. S. and Furbo S. (1996) Optimisation of mantle tanks for low flow solar heating systems. EuroSun 96, Special Issues S Shah L. S. and Furbo S. (1997) Correlation of experimental and theoretical data for mantle tanks used in low flow SDHW systems. Paper presented at ISES 1997 World Congress, Taejon, Korea 6 Proceedings of Solar 97 - Australian and New Zealand Solar Energy Society Paper 101