Evaluation of Convective Heat Transfer and Natural Circulation in an Evacuated Tube Solar Collector

The evacuated tube solar collector ETC is studied intensively and extensively by experimental and theoretical works, in order to investigate its performance and enhancement of heat transfer, for Baghdad climate from April 2011 till the end of March 2012. Experimental work is carried out on a well instrumented collector consists of 16 evacuated tubes of aspect ratio 38.6 and thermally insulated tank of volume 112L. The relation between convective heat transfer and natural circulation inside the tube is estimated, collector efficiency, effect of tube tilt angles, incidence angle modifier, The solar heating system is investigated under different loads pattern (i.e closed and open flow) to evaluate the heat loss coefficient from tank and tubes, test the collector with various aspect ratios (32.9 and 27.2). The enhancement in collector performance is studied by using two reflectors (Flat Plate and Curved Plate) and nanofluid (Water-AL2O3).Theoretical work is run by software (Fluent 6.3), to compute the velocity and temperature profiles within the tube, for different tube diameters, effect of tube junction angle and stagnant region in the bottom of the evacuated tube. The experimental results shows that the heat loss coefficient for tube is W/m.K and for tank is W/m.K, the maximum collector temperature is 79°C in winter and 99°C in summer, while that belong to nanofluid collector is 99°C in winter. The best tilted angle (optimum) of evacuated tube is 41° annually. The collector efficiency increased when using nanofluid of (1, 0.6, 0.3)% volume fraction as(28.4, 6.8, 0.6)% respectively. The efficiency decreases as (33, 62)% when decreasing tube aspect ratio from 38.6% to 32.9% and 27.2% respectively. An increase of (16.9 and 7.08)% in collector efficiency is obtained when using curved and flat plate reflectors respectively. From simulation the best junction angle of the tank is 22.5 ̊. The stagnant region is influenced with changing heat flux, tilted angle and aspect ratio.


INTRODUCTION
ETC evacuated tube solar collector is device of using and utilizing solar energy as water heating system.ETC works with pure natural convection with constant solar heat flux, ETC is important in studying the enhancement of heat transfer and occurs in various industrial solar applications like domestic water heating, space heating, cooling, and solar refrigeration.ETC had been investigated by (Qaiser Muslim Al-asadi 1993) he was investigated ETC performance theoretically by using simple numerical method, (I.Budihardjo, G.L. Morrison and M.Behnia 2003), (John H. Lienhard IV. and John H. Lienhard V. 2008), (Michel Hayek 2009) they had been investigated the ETC experimentally and evaluated ETCs performance and heat enhancement.The whom were investigated The ETC under Baghdad climate experimentally are (Hamza Jabbar Hammad 2009), (Hassan Naji Salman AL-Joboory 2009).The current work interests with evaluation of ETC performance by evaluating convective heat transfer and natural circulation, Moreover, an improvement have done on this performance by using nanofluid (AL 2 O 3 +water) 10nm in diameter of particle with concentrations (0.3%, 0.6% ,1%) of volume, and implementing different kinds of reflector plates (Flat, Curved), tests the ETC at wide range of angles experimentally.Velocity and temperature profiles were founded theoretically with temperature stratification in the tank with help of Fluent 3.6 software.

EXPERIMENTAL WORK
The experimental work was conducted under Baghdad climate using ETC, it consists of 16 tubes deflected at angle β=45° from horizontal, aspect ratio of 38.6, tank 112.1L, and auxiliary heating element, see figure 1. Thermocouples were distributed in ETC, 5 thermocouples are installed at the third top tube opening three of them are acquiring the hot outgoing water from the tube and two of them are acquiring cold in coming water to the tube, 8 thermocouples were installed vertically in the tank, two thermocouples installed at inlet and outlet of water in case of open flow test (Load Test), the ambient temperature was acquired also, see figure 2. Two kinds of reflector plates were used (flat and curved) to achieve more heat input to the system.Same manner of thermocouples distribution has made on nanofluid rig, see figure 3, except the tank has only three thermocouples because it is smaller.Experimental has carried out under Baghdad condition from April 2011 until end of March 2012, the metrological data took from Ministry of science and technology by their data logger.ETC efficiency is investigated also, the reading took during mid day (noon) because the sun will be perpendicular and heat flux maximum, the efficiency was extremely fluctuated due to dependency on temperature difference and precision, it can evaluate from the following equations. ( The results are listed in table 3 which represents the average approximated. Incidence angle modifier (IAM) defines the ratio of the collector output at a given incidence angle and the collector output at normal incidence , see equation 10: The    ETC has been tested at different tube lengths in order to investigate more details on ETC aspect ratios changing, (170cm "standard length", 145cm ,125cm) by putting sand in the tube at limited length and putting a piece of corn above the sand to prevent mixing the sand with water, this have done on all tubes, see figure 7: see figures 19, 25 and 26.The ETC above consists one evacuated tube, tank volume is 4L, total volume including tube is 6.676L, the heat exchanges from nanofluid to fresh water which pumped by electric pump through copper coiled tube (Length = 1.9m,ID = 0.5cm, thickness = 1mm, K=365 W/k.m), see figure 9:

THEORETICAL WORK
Theoretical part has made to support the experimental work, especially the results that could not achieve practically.Although the results restricted with proper assumptions and error that come from simulation, theoretical work describes the ETC simulate using CFD package (FLUENT 6.3).see figure 11.The ETC modeling is ran on one tube and tank volume 121/16 L, a simplification has made (Symmetry condition) on both tank and ETC.see figure 12.

Figure 12: ETC modeling (Symmetry condition)
The modeling has carried out separately on ETC and tank, the condition in junction point between the tank and the tube is the same, the other boundary condition were taken from previous studies see figure 13.
A constant heat flux and constant wall temperature boundary conditions were studied, moreover 2D and 3D were also studied, regarding the natural convection a Boussinesq approximation which states that the density is constant in all governing equations terms except in gravity affect terms, the rest of parameters are taken from experimental work like ( ).
Figure 13: ETC modeling and boundary conditions.
Heat loss from ETC to the environment by various mode of heat transfer, see figure 14: During normal situation solar array incidences on ETC, most of it will utilize and transfer to the fluid, and the rest will be lost to ambient.

+ EXPERIMENTAL RESULTS
Results were obtained experimentally are in following tables and graphs: Heat loss coefficient is evaluated to different tanks, its value is influenced with type of thermal insulation, range of temperature difference and tank volume.See table 1 and figure 17:   The efficiency of ETC is investigated with carious reflector plated with data extremely fluctuated, the results indicated the collector works without reflector is more efficient than with reflector due to the increment in amount of heat input and rising in average collector temperature therefore the amount of heat loss will increase.See table 3: The experimental is indicated the ETC is producing non-uniform flow rate across the collector.See figure 21, depending on solar exposure and effect of heat loss such that the tubes at the edges are receiving more solar radiation once the sun in the horizon than the others, and the temperature of edge tubes (1 and 2) is relatively colder than the other (7 and 8) due to more heat loss, it is near by the tank surface.The optimum tilted angle is approximately estimated for each individual month of Baghdad solar radiation.See figure 24, the results are showed the optimum tilted angle is bigger than the normal angle difference 5˚ to 12˚ and the average annual tilted angle is 41˚ and during the winter season is 53˚.The results which gained from tests on ETC at different aspect ratios indicated the heat loss from bigger aspect ratio more than the others due to wider heat loss surface area, and circulation rate with bigger aspect ratio is more than the others due to the quantity of incidence solar radiation on the tube.See figures 25 and 26.

THEORETICAL RESULTS
The main results were obtained by program (FLUENT 6.3) on ETC as following: figures 27, 28 and 29 are to evaluate heat loss coefficient of tube and tank respectively.Figure 30 shows the best tube tank junction angle velocity contours, which is 22.5˚ from -y axis.This angle gives good temperature stratification in the tank for the current design theoretically.Figure 31 showing the velocity contours of stagnant region, this region is influenced with ETC configuration, angle of inclination from the horizon and amount of solar radiation.Figures 33 and 34 are temperature and velocity contours respectively for cross section aria of ETC at the opened end of the tube (junction with tank).
The average tank temperature of the tank affect on circulation rate if we consider same solar radiation on every case, the warmer average tank temperature produce more flow rate because the viscosity of fluid effects down with temperature increasing, therefore the fluid with higher temperature facing less prevention (less shear) with wall.See figure 35.The simulation of ETC at various diameters is showing in figure 36, the effect of increasing in diameter will increase the heat transfer coefficient, it means there is more heat transfer in the tube.

CONCLUSIONS
-The best method to evaluate is the second method with 24%, because the eliminating of temperature gradient.
-The tilted angles of ETC for Baghdad climate have been investigated and showed the best tilted angles of ETC is relatively more than normal, annual angle is 41˚, during winter is 53˚, during summer is 28˚.
-The efficiency will increase 7.08% with using flat plate reflector, and 16.9% with using curved plate reflector.
-The volume concentration of Al 2 O 3 is proportional to ETC performance, efficiency will enhance 28.4% with 1% of Al2O3, and 6.8% with 0.6% of Al2O3, for 0.3% of Al2O3 doesn't make sensible enhancement.
-An evaluation of heat and mass circulation rate by non dimensional correlations for different conditions.
-(IAM) for different reflectors and shown that the ETC without reflector gives wide range of (IAMs).
-The effect of aspect ratio 27.2 the Re number will decrease to 62%, and aspect ratio 32.9 the Re number will decrease 33% from standard case which is 38.6 aspect ratio.
-The experimental work showed the tubes across collector provide different rates, the third tube in collector represents the equivalent tube across the 16 tube collector.

Figure
Figure 1: ETC assemble
test has carried out using six couples of ETCs each couple connected from top by manifold, three of them mounted on moved frame (sun trucker), see figure4, and the rest of three pairs are seated on the ground and inclined at 45°, PRs were used in wither on sun tracking or tubes on the ground.All the manifolds receive constant flow rate 0.3 L/min during noon from 10:30 to 13:30.The IAM various during the day with incident angle changing and the behavior of evacuated tube as in equation 11 and fig.23.Where C is constant calculated experimentally.

Figure 5 :
Figure 5: ETC couples of tubes at various inclinations.ETC has been tested by open circuit flow (load test) with constant water flow rate (1.1215, 0.6, 0.48 L/min) with almost constant inlet temperature various slightly + 1.5˚C for entire test, this test shows the ETC performance under

Figure 7 :Figure 8 :
Figure 7: Test ETC at different lengths.An enhancement on ETC has been performed by implementing nanofluid (Al 2 O 3 + Water) instead of ordinary fluid Water at different concentrations of nano particles Aluminum

Figure 9 :
Figure 9: Copper coiled tube and tank ports.Nanofluid has been prepared and mixed by (Ultrasonic Cleaner) for 17hrs, eventually no sediments in bottom of device's container were observed.The tests by nanofluid are performed via using the both (plug and loaded test) for entire Al 2 O 3

Figure 11 :
Figure 11: ETC with cylindrical coordinate.ASSUMPTIONS 1-Perfect insulation at the bottom edge of the collector.2-Density is comply to Boussinesq approximation.

Figure 15 :
Figure 15: ETC temperatures section.Figures 15 and16 indicates the ETC thermal net work and its equivalent, the outer glass receive the solar radiation then most of solar radiation passes through the outer glass (Transparent surface) and little reflects as a losses, the other heat loss by radiation and convection, the inner glass (Absorber) absorbs most of the fallen radiation which come from outer glass and transfer it to the liquid inside, as well as it exchanges the rest of heat as a loss with outer glass, the lower surface of absorber has a tank temperature been assumed.

Figure 22
Figure22shows the energy gain from collector discharge temperature during open flow test at different loads.Test showed the load with less mass flow rate produce higher discharge temperature due to maintaining the internal energy in the collector.

Figure 22 :
Figure 22: Energy gain vs. Discharge water Temp.Incidence angle modifier (IAM) is evaluate at various reflector plates, figure 23 shows the effect of reflectors on (IAM) and the amount of constant C in equation 11 at various condition by regression the data.

Figure 24 :
Figure 24: Normal tilted angle and Optimum tilted angle for ETC across year for Baghdad climate.

Figure 25 :
Figure 25: Heat loss from ETC at different lengths vs. time history.

Figure 26 :
Figure 26: Circulation rate vs. solar radiation of ETC at different lengths.
Figures 28 and 29 are showing the temperature gradient elimination to two cases (With and without circulation pump).

Figure 27 :
Figure 27: Evaluation the U tube of ETC.

Figure 28 :
Figure 28: Evaluation the U Tank of ETC First Method

Figure 30 :
Figure 30: Tube junction angle effect on tank velocity profile.

Figure 32
Figure32shows axial velocity profile at top of tube (junction with tank) with different inlet water temperature.The reason of increasing in velocity peck with increasing in temperature is come from more heat radiation fallen on the tube affecting on fluid particles disturbance, logically the amount of heat input is proportional with flow rate and fluid temperature.

Figure 36 :
Figure 36: Heat flux vs. heat transfer coefficient of ETC at different diameters.The effect of nanofluid concentration on circulation flow rate in two constant heat flux subjected on the upper half of ETC wall, figure37shows the simulation results, the increasing in

Figure 37 :
Figure 37: Circulation rate of ETC at different nanoparticle concentration and heat fluxes.

Evaluation of Convective Heat Transfer and Natural Abbas Ahmed Hasan Circulation in an Evacuated Tube Solar Collector
ETC, this relation comes from regression the two variables evaluating the constants a and b in equation 3; Test should run in plug flow test and results are listed in the table 2. See figures 18, 19 and 20.

Figure 17: U tube vs. temperature difference. The
relation between convective heat transfer and natural circulation has been estimated by merging a dimensionless numbers Ra* and Re, by regression the data to evaluate the constants a and b.The results showed that the relation become more proportional with using reflector plates, as well as with increasing the aspect ratio, and as long as the concentration of nanofluid is increased.See table 2 and figures 18, 19, 20: