Category Archives: EuroSun2004

Mathematical Model

The cross-sectional view of the used mechanical model for the thermal stratifications for numerical analysis is shown in Fig. 1.

Boundary layer equations were used to determine the temperature and velocity distributions in the flow field. The analysis was based on the unsteady, three-dimensional continuity, momentum and energy equations.

The assumptions are used in the present study as:

— The flow is unsteady, turbulent and three-dimensional,

— The thermal conductivity of the tube sheet material does not change with temperature

— The tube material is homogeneous and isotropic.

Three dimensional continuity, momentum and energy equations are solved numerically. The upwind and central difference method used for convections and diffusions, respectively [9].

The hot water temperature is desired to keep constant as long as possible in the upper part of the tank. Therefore, different obstacles are placed into the tank in order to supply higher thermal stratifications. The schematic view of the obstacle type and geometries are shown in figure 2. The details of these geometries are shown in figure 3. Obstacle types and tank matches are

Table 1 Obstacle types and tanks matches

Obstacle types

The obstacle placed tank models.

1

7,8

2

9,10

3

12

4

11

5

12

6

1,2,3,4,5,6

The water temperatures are assumed 333, 320 and 285 for water exit from the tank to usage, water in the tank and water coming from the main lines, respectively.

V.

Figure 1. Cross-sectional view of the used tank model

j________________ ______________________ 1______________ ______________________ L

1___ . .__

__ 2___

і

. 3 _____

4

11 ‘

.. •■***’*•. ..

……………………

• ‘ 1 ‘ n

5

6

9

10

11

12

Figure 2 Obstacle geometries and its assembly shape in the tank.

The dimensions of the obstacles in figure 3 are shown in Table 2. Table 2 The dimensions of the obstacles

Engel no

a

s (m)

t (m)

r1 (m)

r2(m)

R. i(m)

r4(m)

l (m)

r5

1

0.8

0.02

0.96

0.2

2

0.8

0.2

0.96

0.0

3

0.2

1.0

4

0.2

1.0

0.2

5

0.2

0.8

6

20“

0.02

0.8

EXPERIMENTAL EQUIPMENT

The hybrid system consisted of a PV module, where a glass plate window (jacket) was layed over the front surface, a circulating pump and a heat exchanger. The glass plate was temporarily attached and could be removed at any time without much effort. A fluid, distilled water, was circulated between the front surface of the PV module and the jacket layed on the front surface.

The incident solar radiation first passed through the glass jacket and the layer of circulating water before reaching the module. The high transmittance of glass and water layers for the visible radiation, ensured that the silicone cells of the PV module would receive most of the visible radiation to generate electricity without much sacrifice in power. Infra-red part of the incident radiation is essentially absorbed by the glass-water and module structure combination leading to warming of the solar cells. It is this excess heat that the hybrid unit is designed to remove, by circulating water, and utilize in domestic applications. While the circulating water removes the heat and leads to cooling of the module, at the same time it provides a source of warmed water for preheating applications in a domestic use.

A standard commercial module, M, rated at 55 Watt peak, with dimensions 130x47x5 cm was used. The jacket, J, had the diamensions of 130x47x1.5 cm (Fig. 4) and was placed over the module surface with a gap, about 0.4 cm to allow flow of water. The PV module was tilted 45 degrees to horizontal and faced south for maximum solar insolation, I. Initially pure water, Q, was circulated at the rate of 36 liters per hour. An identical PV module was used as a control and measurements were performed on both the hybrid and the control modules at the same time and location.

A data acquisition card was used to record a range of data. Meteorological information published by the Statistics office[1] was used. The electrical characteristics of both modules and thermal performance of the hybrid system were measured. The data listed below were recorded at various times from 9.00-17.00 hrs. each day and the average values were plotted. The temperature of the circulated water, at input/output Tin, Tout, the ambient temperature, Tamb, and surface temperatures, Tsc, Tsh, on the control and hybrid modules respectively were recorded. All temperatures were recorded in degrees centigrade. The short-circuit currents, Iscc, Isch and open-circuit voltages, Voc, Voh of both control and hybrid modules respectively were recorded. The maximum operating power of each module is defined as P = Voc Iscc FF (1)

The fill factor, FF, was taken as 0.7, which is typical of single crystal silicone cells.

The electrical energy, Ec and Eh, of control and hybrid module respectively were calculated using numerical integration. Hence, the area under the power-time curve, was taken at various one-hour periods.

Thermal energy collected by the circulating water in one-hour period, can be defined as Qw = Q Cp AT (2)

where Q is the volune flow rate of the water in lt/hr, Cp is the specific heat at constant pressure (4187J/kg. °C), and AT is the temperature difference (Tout — Tin ).

Temperature distributions according to the different tank models

The obstacle types, have supplied higher thermal stratification, have been determined numerically by using computer simulation. In this situation, the aim is supply hot water as long times as possible with high thermal stratification between the temperatures T4 (cold water enter to the tank from main lines) and T2 (hot water supplied by solar collector). Different kinds of obstacle as in figure 3 with different assembly as in figure 2 have been placed in the tank in order to obtain this higher thermal stratification. These obstacles are placed in the tank with suitable angles and coordinates to decrease contact area between cold and hot water in the tank. T4 flow fibers don’t have to destroy hot water gradient. However, the hot water entering the tank (T2) would destroy the stratification. Therefore, hot water velocity is taken as 0.2 m/s for forced circulation. The flow rate of the hot water from tank to usage must be equal to the cold water flow rate entering to the tank. Therefore, the cold water velocity is assumed as 1 m/s.

Vortexes have occurred in the tank because of the mixing of two different fluids. These two fluids would hit each other towards to the wall and obstacle surfaces. Therefore, calculations are carried out by using wall functions method and standard k — s turbulent models with FLUENT [10] code.

The cold water enters and exit pipes in radial direction are in the same axis as symmetrical condition. So, higher percentage of the cold water entering to the tank would be directed towards to cold water exit channel (T1) from the tank. At the end, the cold water can not produce a lot of vortex inside the tank and destroying effects into thermal stratification inside the tank would be decreased. The hot water exit channel (T3) is generally located at the top of the tank. So, the hot water can be supplied as long as possible because of the positions of the hot water exit channel.

NUMERICAL ANALYSIS OF THE EFFECT OF USING SOME. OBSTACLES INTO THERMAL STRATIFICATION IN HOT. WATER TANK WITH NATURAL CONVECTION

Necdet ALTUNTOP1, Mevlut ARSLAN1, Veysel OZCEYHAN1,

1 Dept. of Mechanical Engineering, Erciyes University, 38039 Kayseri, Turkey

Abstract

In this work, the numerical analysis of the effects of using different obstacles into the thermal stratifications in the cylindrical hot water storage tank has been presented. This kind of storage tank is widely used in heating systems by solar energy. The heat transfer is carried out by natural convections in the tank. The water is used as fluid. The Optimal shape and angle of the obstacle has been found to obtain higher thermal stratification as possible as between cold and hot water between 30 different obstacles. The natural convection effect into the temperature distribution inside the tank has also obtained for different obstacle types and angles. The temperatures of the water entering to the collector and from the collector and water exiting form the tank to usage have been presented into the graph. The temperature differences of these waters have also presented to determine thermal stratification according to the obstacle types and angles. The temperature distributions in the tank between with and without obstacle have been analyzed to compare to get high thermal stratification. At the conclusion, It is found that the obstacle type 11 has supplied the highest thermal stratification between all investigated cases.

Conclusions

The temperature distributions of the heat storage tank with different obstacles are calculated numerically and presented in z-r plane. Temperature distribution of the smooth tank is also presented. The results as follows;

Using the obstacles are improving the thermal stratification inside the tank compared to the smooth tank.

It can be observed that, the obstacle types have gap in the center, have better thermal stratification than obstacle types that have gap near the tank wall.

Obstacle types 7 and 11 have supplied hot water with higher degree to usage rather than other obstacles type and also rather than smooth tank. This is desirable case for thermal stratifications. Other obstacle types have little effect into the thermal stratifications in the tank. The smooth tank has also little thermal stratifications.

While the operation periods of time, the tank has obstacle type 11 has lower average value of temperature in the z=0.2 plane in radial direction than the tank has 7 type of obstacle. The reason of this case is that tank model has obstacle type 11 has cylindrical shaped obstacle in z=0.2. This obstacle prevents the destroying effect of the cold water into the thermal stratifications while the operation period of time increases. So, in tank 11, the temperature of the water going to the heater would decrease while the time period increases. This is also desired situation for heat storage tank in solar energy systems.

T3 has increased in tank 11 and T1 has decreased during the operation times. So, the difference between T3 and T1 would also increase. This is also desired criteria for thermal stratifications. With respect to these arguments, the tank has obstacle type 11 has best thermal stratification between all investigated cases.

The tank has obstacle has improved the solar collector efficiency as well as thermal stratifications. Because, T1 (water return to the solar collector) would decrease when the obstacle is used. So, the increase of the solar collector efficiency can be achieved.

Acknowledgement

The authors thank to the Erciyes University for FLUENT 6.1.22 code.

Theoretical Model

The cross-sectional view of the used model for numerical analysis is shown in Fig. 1.

Boundary layer equations were used to determine the temperature and velocity distributions in the flow field. The analysis was based on the unsteady, three-dimensional continuity, momentum and energy equations.

The assumptions are used as:

— The flow is unsteady, turbulent and three-dimensional,

— The thermal conductivity of the tube sheet material does not change with temperature

— The tube material is homogeneous and isotropic.

Three dimensional continuity, momentum and energy equations are solved numerically. The upwind and central difference method used for convections and diffusions, respectively [9].

The aim of using the obstacle is supplying hot water as long as possible in the upper part of the tank to usage. So, high thermal stratification would be achieved by using these obstacles to obtain higher degree of hot water from the tank. The obstacle’s schematic view is shown in figure 2. The details of these obstacle geometries are shown in figure 3. Table 1 indicates that the matches between obstacle type and tank.

d =0.02 m f1 = 0.04 m Vk=0.2 m/s

D= 1 m g = 0.2 m

f = 0.2 m Si = 0.2 m

The hot water entrance velocity is assumed as 0.2 as forced convective regions. The water temperatures are assumed 333, 320 and 285 K for water exit from the tank to usage, water in the tank and water coming from the main lines, respectively.

dl

1

T4

V

Figure 1 Cross-sectional view of the used tank model

Table 1 Obstacle types and tanks matches

Obstacle types

The obstacle placed tank models.

1

7,8

2

9,10

3

12

4

11

5

12

6

1,2,3,4,5,6

The dimensions of the obstacles in figure 3 are shown in Table 2.

Table 2 The dimensions of the obstacles

Obstacles number

a

S (m)

t (m)

r1 (m)

r2(m)

r3(m)

r4(m)

l (m)

r5

1

0.8

0.02

0.96

0.2

2

0.8

0.2

0.96

0.0

3

0.2

1.0

4

0.2

1.0

0.2

5

0.2

0.8

6

20o

0.02

0.8

2.1 Physical properties of fluid and boundary conditions

The thermodynamic properties of the water are considered in this study. The velocity and is assumed to be zero at the beginning. The operation pressure in the tank is taken1 atmosphere and the hot water temperature is assumed to be 320 K when the water usage started. The time between the water is started to be used and the water usage was finished is assumed as

calculation region. This time is taken as 30 minutes and the problem solved as unsteady.

Figure 3 The details of the obstacles in the tank.

INVESTIGATIONS OF THE SOLVIS STRATIFICATION. INLET PIPE FOR SOLAR TANKS

E. ANDERSEN, U. JORDAN, L. J. SHAH, S. FURBO Department of Civil Engineering, Technical University of Denmark Building 118, DK-2800 Kgs. Lyngby, Denmark Tel.: +45 4525 1901, Fax: +45 4593 1755, ean@byg. dtu. dk, uj@byg. dtu. dk, ljs@byg. dtu. dk, sf@byg. dtu. dk

Introduction

Since the 1960’ties the influence of the thermal stratification in hot water tanks on the thermal performance of solar heating systems has been studied intensively. It was found, that the thermal performance of a solar heating system is increasing for increasing thermal stratification in the hot water tank.

The temperature of the storage water heated by the solar collector loop usually varies strongly during the day. In order to reach a good thermal stratification in the tank, different types of pipes, plates, diffusers and other devices have been investigated in the past (e. g. Loehrke, 1979). The aim pursued was to transport the heated water into the tank level of corresponding temperature.

Flexible stratification pipes (manifolds) have been further developed for example by (Gari et al., 1982). Furthermore, a wide variety of non flexible tubes with either open holes and perforated vertical plates inside the pipes (Davidson, 1992) or openings in form of balls (e. g. Leibfried, 2000) or flaps (e. g. described in Krause, 2001) have entered the market during the recent years.

In this paper an investigation of a stratification pipe with openings covered with flaps according to (Krause, 2001) is presented. The flaps are constructed with a soft material which allows the flap to close and open depending on the temperature and pressure differences inside and outside the pipe. Figure 1 shows schematic

illustrations of the pipe. The total height of the pipe is 328 mm, the outer diameter 60 mm, and the flaps are located with a distance of 292 mm in vertical direction (distance between the centre of each opening).

Preliminary laboratory tests by (Shah, 2002) with the same stratification pipe containing 5 openings showed that thermal stratification was well built up for a volume flow rate smaller than 8 l/min and larger than 4 l/min, regardless of the inlet temperature, the temperature level in the tank, and the thermal stratification in the tank. For volume flow rates larger than 8 l/min, however, the number of open flaps increased, so that
water entered the tank at different levels instantaneously. For volume flow rates smaller than 4 l/min laboratory tests indicated that cold water could be sucked in through an opening in a low level due to low pressure differences. The cold water that entered the pipe through these openings from the bottom of the store mixed with the heated water that flew through the pipe and thereby induced mixing in the tank during charging.

More detailed investigations of the flow structure close to the flaps of the stratification pipe are presented in the following for one set of operating conditions. Temperature measurements were carried out and an optical method called Particle Image Velocimetry (PIV) was used to visualize the flow around the flaps.

Experiments

Experimental Set-up

The experimental set-up is shown in Fig. 2. The set-up consists of a rectangular glass tank with side lengths of 400 x 400 x 900 mm3, a heating and a cooling unit, and standard PIV equipment (Dantec Dynamics). The PIV equipment consists of a laser, a camera and a processing system for analysing the pictures taken by the camera. Information about the PIV equipment is given in Table 1.

Table 1. PIV equipment.

laser

type

energy/pulses

wavewlength

Nd:YAG, NewWave Solo (Neodym-Yttrium-Aluminium — Granat)

100 mJ/pulse

532 nm (frequency doubled)

CCD

type

HiSense 12 bit

camera

resolution

1280 x 1024 pixel (64 x 64 pixel interrogation area)

particles

Polyamid, 5pm (PSP-5)

software

Flowmanager, Dantec Dynamics

The camera is placed perpendicular to the laser that illuminates a thin slide in the flow.

The inlet consisting of three compound stratification inlet pipes placed in the centre of the tank. The inlet pipe is closed at the top. The outlet is placed in the bottom of the tank in the corner behind the inlet pipe. The temperature is measured in the middle of the pipe below each inlet and in 13 uniformly distributed levels in the tank. Also the in — and outlet temperatures are measured as well as the volume flow rate. The temperatures are measured with thermocouples type TT with an accuracy of 0.5 K. The volume flow rate is measured with an electro magnetic inductive flow meter, type HGQ1 from Brunata HG a/s. The flow meter has an accuracy of about ± 1 %.

The numerical method

CFD code is used to obtain velocity and temperature distributions inside the tank. Three dimensional unsteady solutions are taken by using implicit method in segregated solver. The discretization is achieved by using standard method for pressure and by using first upwind method for momentum, energy, turbulent kinetic energy and turbulent dissipation rate equations. The SIMPLE (semi-implicit method for pressure-linked equations) algorithm was used for flow calculations [10].

Te under-relaxation factors are assumed as 0.3, 0.7, 1, 0.8, 0.8 and 1 for pressure, momentum, energy equations, turbulent kinetic energy (k), turbulent dissipation rate (s) and turbulent viscosity (pt) as, respectively. This factor has assumed as 1 for body force and density.

The convergence factors are used as 10-3 for continuity, momentum, turbulent kinetic energy and turbulent dissipation rate equations. This value is taken 10-6 for energy equations [10].