Ulrike Jordan, Simon Furbo

Technical University of Denmark, Dpt. of Civil Engineering
DK-2800 Kgs. Lyngby
Tel.: 0045-4525-1889, sf@byg. dtu. dk

Abstract: Experimental and computational investigations of the flow fields around buffer plates in a small domestic hot water tank are presented. Inlet devices with different buffer plate diameters were placed at the bottom of an experimental glass tank. Temperatures were measured in different tank levels and two-dimensional velocity fields were measured in the centre plane of the tank with an optical method called Particle Image Velocimetry (PIV). The experimental results were used to model the influence of the buffer plate diameter on the stratification in the tank. The model is suitable for a limited range of buffer plate diameters. It was implemented into the simulation tool TRNSYS. Annual system simulations of a typical small solar domestic hot water system confirm earlier findings that the net utilized solar energy of the investigated and typically marketed system in Denmark could be improved by about 3 to 5 percent by employing a suitable buffer plate.

1. Introduction

In Denmark small solar domestic hot water systems typically consist of smaller collector areas and storage tanks than in Central Europe. The volume of the storage tanks is usually about 180 litres and the pipes are connected through the bottom of the tank. A buffer plate is placed above the vertical inlet pipe for cold water. Previous studies about stratification in solar domestic water tanks showed already that the design of buffer plates plays an important role for the system performance, especially if water is drawn off the tank with high flow rates. For example, (Carlsson 1993) carried out experiments with four different cold-water inlet devices (direct inlet, bent, perforated pipe and parallel plates) connected through the tank side of stores with a volume of 2 m3. A horizontally bent tube (facing in tangential direction to the side) turned out to cause the whole water volume in the tank to circulate in the direction of the entering fluid during and after a draw-off. In general, a large cross sectional area of the inlet was found to be of advantage. (Huhn et al. 2002) carried out experiments with inlet pipes entering the tank from the tank bottom and determined characteristic numbers for a tank with an open pipe, a bent tube and two buffer plate inlets. These numbers, however, could not directly be used to predict the system performance reduction or increase due to the buffer plates.

Investigations of the thermal stratification in solar stores with Particle Image (or Tracking) Velocimetry (PIV) were carried out, e. g. by (van Berkel 1997), (Shah, 1999) and (Knudsen et al. 2003). PIV is an optical experimental method to visualize velocity fields in a fluid. Van Berkel investigated the case of a flow stream entering the tank from the side with high velocities. The investigations were focused on the thickness of the thermocline situated between two initial temperature layers in the tank. Shah and Knudsen visualized the flow fields of different mantle tanks to determine thermal stratification inside the mantle as well as inside the tank.

Previous to the presented study, a model has been developed, based on measurements with two differently shaped buffer plates built into marketed steel tanks (Jordan and Furbo 2003-1). Based on the previous study, a further development of the model is described in the following, containing a relation used to quantify the impact of the buffer plate diameter on the system performance of a solar heating system.

2. Experimental Set-up

A scheme of the experimental set-up is shown in figure 1. It consists of a glass tank with a water volume of about 136 litres. Cold water enters the bottom of the tank and warm water is drawn from the top. The inlet — and outlet-pipes are connected to a cooling unit, a heating unit and to a buffer tank.

Temperatures are measured at 14 different levels of the tank. Instantaneous values are captured in time intervals of 10s. The thermocouples (of type TT) are mounted close to a tank corner. Average flow rate values over a time interval of 10s are measured with a magnetic-inductive flow meter.

The Particle Image Velocimetry — (PIV) equipment consists of two lasers, a camera, a processor unit (used to trigger camera and laser, as well as for data processing) and a computer (for further data processing).

PIV is a non-intrusive optical method to measure two — or three-dimensional velocity fields in a fluid. Small tracer particles (with a diameter of 20pm) are added into the fluid and illuminated by a pulsed laser sheet. The scattered images of the particles are recorded with a camera, based on electronic solid-state imagers (charge couples device (CCD) cameras). The time delay between two laser pulses is being adapted to the mean velocity of the flow and the magnification at imaging. It is assumed that the tracer particles move with the local flow velocity between two illuminations.

Measurements were carried out with three different buffer plates shown in figure 2.

Uniform initial tank temperatures at three different levels (of about 28°C/31°C, 42°C and 57°C) and up to four different flow rates were applied as reference conditions for the experiments of each inlet device. The inlet temperatures varied between about 7 and

Fig. 1: Experimental set-up. Water volume in the tank: 1361; side length of the tank: 0.4 x 0.4 x 0.9 m3, PIV-laser-system (Nd-YAG), camera type: HiSense. Field of view for the camera: about 200×160 mm2 (rectangle).

Fig. 2: Investigated inlet devices. Max. diameters: 28, 52, and 70 mm; height of the buffer plates 12, 18, and 26 mm; widths of inlet gaps: 10,10, and 20 mm respectively.

3. Measurements

The flow field around the buffer plates is influenced significantly by the conditions applied. To investigate the effect of solely the flow rate and the plate diameter on the velocity field, in a first step, ambient temperature was used for both the water inside the tank and the

entering water. The field of view is marked by a rectangle shown in figure 1. It reaches from the centre of the tank to the tank wall on the right side of the tank, in a vertical pla The inlet devices are placed in the bottom centre of the field of view.

As an example, velocity vector maps, measured with a flow rate of 4 l/min, applying th small, medium scale and large buffer plate, respectively, are shown in figure 3. The velocity vector maps show mean values of 100 instantaneous vector maps, measured time intervals of 10 s to 200 s (time between recordings: 0.25 s to 2 s).

As shown in figure 3a, the water entering the store through the small inlet device pass the device with a fairly high vertical velocity component, with an angle of the flow direc to the vertical of about 45°. In contrary, with the medium scale inlet device and the sar flow rate (figure 3 b), the entering water is first directed to the tank bottom, then strean the wall, and from there in vertical direction. For the large inlet device, the water is also first directed to the tank bottom. However, no steady flow direction can be specified frc the vector map of mean values, due to the low velocities of the flow.

In figure 4 the vertical velocity component vz of a measurement with the small inlet dev and a flow rate of about 8 l/min is shown for six different storage levels, between h = 40 mm and 140 mm. The position of the lower outlet height of the opening of the smal inlet device is placed at h = 40 mm (see figure 2). As expected, the maximum value of situated in the curve closest to the inlet gap (at h = 60 mm), with a value of about 0.12 At h = 120 mm, the maximum value dropped by about two thirds.

In order to quantify and compare the upward velocities for different flow rates, the mea square roots of vzm in the horizontal was calculated for 4 measurements with different rates, uniform temperatures, using the small inlet device. vzm is defined as:

with N = 39 velocity vectors in horizontal direction

As shown in figure 5, the peak of vzm(h) moves only slightly upwards to increased stor heights for increasing flow rates. This means that the angle of the entering flow directs the vertical is approximately independent of the flow rate for the given reference conditions.

The strong deviation between the curves for a flow rate of 12 l/min shows that the flow was not stationary and that more images maps needed to be captured in order to get statistically independent results. The difference between the inlet temperature and the temperatures in the storage tank was within the measurement accuracy band of 0.3 K If the temperature of the water entering the tank is smaller than the temperature of the water in the tank, buoyancy effects occur for the flow stream. The cold water is presse downwards and the mixing is reduced. Therefore, the stratification of the tank is impro

the higher the temperature difference between the water in the bottom part of the store and the entering water.

In figure 6 and 7 measurements of the temperature distribution and velocity fields captured during a draw-off are shown. The large buffer plate was applied. Figure 7a) shows storage temperatures at different heights, the inlet temperature and the flow rate over the time. The initial tank temperature was 57°C, the flow rate about 11 l/min, and the inlet temperature about 7°C. The two temperatures measured at the lowest tank heights (h = 20 and 60 mm) are approximately equal throughout the measurement. The slope of these two curves is relatively small, which corresponds to a high degree of mixing of the lower part of the store at the beginning of the measurement. Nevertheless, the slope of the curves increases during the draw-off, for the (later) temperature drop of higher temperature layers. The size of the boundary layer that remains after the draw-off can be regarded as relatively small. The temperature gradient is higher than 30 K / 60 mm.

In Figure 6 a-d PIV velocity vector maps are shown. Each vector map shows mean values of 10 instantaneous vector maps. The corresponding particle images were captured within a time interval of 2.5 seconds. The cold water streams from the centre of the tank towards the tank wall. Next to the wall a vortex is developed, that rises throughout the draw-off.

Measurements of the thermal stratifications were carried out for draw-offs of a volume of about 40 l with three different (uniform) initial tank temperatures and four different flow rates for the three inlet devices. As an example, the distributions of the relative temperatures in the tank with the small buffer plate are shown in figure 8. The relative tank temperature is defined as:

T — T with: T : temperature at a given storage height

Trel = Tm : temperature of entering cold water

Tmax _ Tin Tmax: mean initial tank temperature

Fig. 7: Large inlet device (d = 70 mm).

7a) red temperatures and flow rate over the time. Mean temperature of the

entering water: 7°C, mean flow rate: 11 l/min, initial storage temperature: 57°C, draw­off volume: 401. The bars show (roughly) the time intervals in which PIV image maps were captured.

7b) Calculated temperatures (TRNSYS) over the time. Input values to the simulation Measured flow rate and inlet temperature as shown in a), as well as the initial storage temperature. Simulation time step: 4.5s.

7c) Inlet height of the water entering the store used for TRNSYS simulations over the time. hin = 55mm (At = 10s), 45mm (1min), 110mm (2min) and 150mm (3min).

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