DOI : 10.17577/IJERTV13IS050286
- Open Access
- Authors : Adebola Oluwafunmilayo, Ibikunle
- Paper ID : IJERTV13IS050286
- Volume & Issue : Volume 13, Issue 05 (May 2024)
- Published (First Online): 28-06-2024
- ISSN (Online) : 2278-0181
- Publisher Name : IJERT
- License:
This work is licensed under a Creative Commons Attribution 4.0 International License
Boiling Heat Transfer and Circulation Rate in a Thermosyphon Apparatus
Adebola Oluwafunmilayo, Ibikunle School of Engineering University of Aberdeen
Abstract – This study examines the boiling heat transfer and circulation rate characteristics in a Thermosyphon apparatus. For the test a Thermosyphon rig with the Reboiler (Evaporator section) and Condenser was used. The heat transfer section (Reboiler) contains a steam-heated stainless-steel rig/tube of 50mm diameter and height of 400mm with distilled water as the working fluid. The aspect height was maintained at 0.73m. It was found that when natural circulation occurs in a thermosyphon apparatus, the heat transfers and circulation rates were highly dependent on the pressure and temperature difference.
The experimental results for operating pressures of 1bar (g) –
1.8bar (g) have shown data sets of Heat load values in the range of
828.7 2371 J/s, heat transfer coefficients values of 550 1221Wm- 2K-1, temperature difference value 22 – 33oC and heat flux values of 13.2 37.6 KWm-1K-1.
Keywords: Boiling heat transfer Thermosyphon apparatus, Natural – circulation, Aspect height, Heat transfer Coefficient.
I.INTRODUCTION
A two-phase closed thermosyphon Reboiler is a high- performance heat transfer device used when a large amount of heat is to be transferred at a high rate when a small temperature difference is required. Its a closed loop filled with working fluid (distilled water). The heat transfer is obtained by the evaporation of the working fluid in the Reboiler (evaporator section).
The thermosyphon Reboiler is a closed vertical tube filled with working fluid (distilled water). Heat is supplied to the Reboiler via steam, which vaporizes the working fluid after some time. Vaporization at the surface of the working fluid occurs due to the heat absorbed by natural convection from a hot rig beneath the surface. There is a transfer of heat from the Reboiler to the condenser in the form of latent heat of vaporization with a small temperature difference.
The two-phase closed thermosyphon apparatus that was worked upon consists of four major components:
The Reboiler (Evaporator) The Rising tube
The Condenser The Falling tube
-
EXPERIMENTAL PROCEDURES
The following procedures were carried out on the thermosyphon apparatus to establish the objectives.
Before the start of the experiment, a test run was done on the thermosyphon apparatus to ensure that its in proper working condition.
After the test run, the aspect height of the working fluid in the reboiler was established. The steam valve was opened to admit steam into the reboiler from the steam mains. The steam pressure was set to 1barg [Different steam pressure values were used]. Following the set-up of the steam and cooling water, the rig was allowed to reach a steady state before experimental run data were collected.
The cooling water flowing from the condenser was collected and measured with time in a measuring cylinder of 2 liters, and the volumetric flow rate was determined.
Collect and log in on logbook values of cooling water inlet and outlet temperatures;
Temperature value on the probe thermometer attached to the bottom of the evaporating column.
The temperature readings were taken at 1min intervals. Rotameter readings [The Flow 1 and Flow 2] were taken at 10secs intervals.
Collect and weigh the steam condensate at 10-minute intervals. The experiment was repeated at various steam pressures (i.e., 1.2, 1.4, 1.6 bar g) and different working fluid aspect heights in the evaporating column. The system was allowed to reach state before new data were recorded. The information was used in conjunction with the reboiler/condenser temperatures to establish the result for the Research.
Fig. 1 : Thermosyphon apparatus
-
RESULTS AND OBSERVATIONS
-
EXPERIMENTAL AND CALCULATED RESULTS
Table 1: Experimental and calculated data for Aspect height of 0.73m
Aspect Height = 0.73m Steam pressure = 2.0bar a
TR
Tin
Tout
FL 1
FL 2
Latent
Heat(L)
Mass
Conde (C)
Heat
Load(Q)
Area
Heat
Coeff (U)
Heat
Flux (F)
T
Time
oC
oC
oC
L/hr
L/hr
KJ kg1
kg/s
W
m2
W/m2oC
KWm2
oC
(sec)
98.0
18.5
39.5
1.5
1.0
2202.0
3.67E-04
828.7
0.063
592.52
13.50
22 .2
0
96.5
18.5
42.0
1.0
1.0
2202.0
3.67E-04
828.7
0.063
555.02
13.50
23.7
60
98.0
18.5
42.0
0.9
2.0
2202.0
3.67E-04
828.7
0.063
592.52
13.50
22 .2
120
96.5
19.0
41.0
1.0
1.0
2202.0
3.67E-04
828.7
0.063
555.02
13.50
23.7
180
93.5
18.5
41.5
1.0
1.0
2202.0
3.67E-04
828.7
0.063
492.66
13.50
26.7
240
100.0
19.0
41.0
1.2
1.0
2202.0
3.67E-04
828.7
0.063
651.19
13.50
20.2
300
98.0
18.5
42.5
1.4
1.0
2202.0
3.67E-04
828.7
0.063
592.52
13.50
22 .2
360
99.0
19.5
41.0
1.5
6.0
2202.0
3.67E-04
828.7
0.063
620.47
13.50
21.2
420
97.0
17.0
43.0
1.4
1.0
2202.0
3.67E-04
828.7
0.063
566.98
13.50
23.2
480
103.0
17.0
44.0
1.5
1.0
2202.0
3.67E-04
828.7
0.063
764.77
13.50
17.2
540
100.5
17.0
42.0
1.3
1.0
2202.
3.67E-04
828.7
0.063
667.71
13.50
19.7
600
Aspect Height = 0.73m Steam pressure = 2.2bar a
100.0
11.0
14.5
1.5
1.0
2193
1.17E-03
1020.6
0.063
695.28
16.2
23.3
0
102.0
11.5
14.5
1.0
4.0
2193
1.17E-03
1020.6
0.063
760.56
16.2
21.3
60
97.0
11.0
14.5
1.5
4.0
2193
1.17E-03
1020.6
0.063
615.97
16.2
26.3
120
100.0
11.0
15.0
2.0
6.0
2193
1.17E-03
1020.6
0.063
695.28
16.2
23.3
180
90.0
11.0
14.5
1.8
1.0
2193
1.17E-03
1020.6
0.063
486.49
16.2
33.3
240
98.0
11.5
15.0
2.2
2.0
2193
1.17E-03
1020.6
0.063
640.32
16.2
25.3
300
101.0
11.0
14.5
1.0
4.0
2193
1.17E-03
1020.6
0.063
726.46
16.2
22.3
360
98.0
11.0
14.5
2.0
10.0
2193
1.17E-03
1020.6
0.063
640.32
16.2
25.3
420
98.0
11.0
14.0
1.5
10.0
2193
1.17E-03
1020.6
0.063
640.32
16.2
25.3
480
100.0
11.0
14.0
1.5
10.0
2193
1.17E-03
1020.6
0.063
695.28
16.2
23.3
540
90.0
11.5
13.5
2.0
22.0
2193
1.17E-03
1020.6
0.063
486.49
16.2
33.3
600
Aspect Height = 0.73m Steam pressure = 2.4bar a
99.0
11.0
16.0
2.0
40.0
2185
1.5E-03
1618.9
0.063
948.22
25.7
27.1
0
99.0
11.0
16.0
0.5
46.0
2185
1.5E-03
1618.9
0.063
948.22
25.7
27.1
60
99.0
10.5
16.0
3.5
38.0
2185
1.5E-03
1618.9
0.063
948.22
25.7
27.1
120
98.0
11.0
17.0
1.5
40.0
2185
1.5E-03
1618.9
0.063
914.48
25.7
28.1
180
99.0
11.0
16.0
3.0
32.0
2185
1.5E-03
1618.9
0.063
948.22
25.7
27.1
240
95.5
11.0
16.5
0.2
46.0
2185
1.5E-03
1618.9
0.063
839.77
25.7
30.6
300
96.0
11.0
16.5
3.0
40.0
2185
1.5E-03
1618.9
0.063
853.72
25.7
30.1
360
95.0
11.0
16.0
3.0
40.0
2185
1.5E-03
1618.9
0.063
826.26
25.7
31.1
420
98.5
11.0
16.5
0.3
45.0
2185
1.5E-03
1618.9
0.063
931.04
25.7
27.6
480
99.5
11.0
16.5
4.0
38.0
2185
1.5E-03
1618.9
0.063
966.05
25.7
26.6
540
99.0
12.0
16.5
2.5
30.0
2185
1.5E-03
1618.9
0.063
948.22
25.7
27.1
600
Table 2: Average calculated values at aspect height of 0.73m
Pressure (Bar a)
Heat load (W)
Heat flux (KWm-2)
Overall heat transfer coefficient
(Wm-2oC-1)
Rate of Circulation (m3/s)
T (oC)
2.0
828.70
13.50
604.70
6.12E-04
22.00
2.2
1020.60
16.20
643.90
1.90E-03
25.70
2.4
1618.90
25.70
915.70
1.03E-02
28.10
-
EVALUATIONS: HEAT LOAD
The heat load of the thermosyphon apparatus was calculated using three different methods. According to theory, it is expected to obtain the same result, but was not achieved from the experimental data due to different sources of error and disturbances that occurred
Method 1
Q = Flow 1 × latent heat of vaporization of water at 1bar (absolute) FL1 = 3.67× 10-04 kg/s
Latent heat of vaporization (from steam table) = 2258KJ/kg
Q = 3.67× 10-04 × 2258 × 1000
Q = 828.7 Watts
Method 2
Q = m× Cp × T m = 5.193E-03kg/s Cp = 4.219KJ/kg K
T (Temperature difference between the cooling water) = 26.4 Q = 5.193×10-03 × 4.219 × 1000 × 26.4
Q = 578.4 Watts
Method 3
Q = Mass flow rate of condensate(C) ×latent heatof vaporization at steam pressure
Mass flow rate = 8.33×10-04kg/s
Latent heat of vaporization at 2bar (absolute) = 2202KJ/kg Q= 8.33E-04 × 2202
Q= 1834.3 Watts
For this research, the Q obtained from the first method will be accepted and used for other calculations because its about the most accurate result.
The Q obtained from method 2 is smaller than that obtained from method 1 due to heat loss by convection; the apparatus set-up had the majority of the pipe uninsulated.
The Q obtained from method 3 is greater than in methods 1 & 2. This implies that the steam supplied is wet – steam, i.e., the pipe through which the steam travels is insulated; hence, some condensation has occurred in the pipe. This affects the accuracy of the condensate flow rate measured during the experiment because there is an existing condensate in the steam earlier. In turn, the heat load (Q) calculated using that in-accurate value would be inaccurate.
Area of the heating ring
Area = d L
d = 0.05m L = 0.4 m
A = 3.142 × 0.05 × 0.4 A = 0.063 m2
IJERTV13IS050286
(This work is licensed under a Creative Commons Attribution 4.0 International License.)
Heat flux
Heat flux = Q/A
Heat flux = 828.7 /0.063 Heat flux = 13.2 KWm-2
Overall heat transfer Coefficient (U)
Q= U A T U = Q/A T
U = 828.7 / (22.0 × 0.063) U = 604.7 Wm-2 ºC-1
Temperature difference (T)
T = Tsteam pressure T(avg)Reboiler unit
T = (120.2 – 98.2) ºC
T = 22.0ºC
-
-
RESULTS
The relationship between different parameters is shown in the graphs below.
Fig 1: Overall heat transfer Coefficient against Temperature difference at Aspect height of 0.73m
Fig 2: Overall heat transfer coefficient (Wm-2oC-1) against Heat load (W)
-
DISCUSSIONS
A. OVERALL HEAT TRANSFER COEFFICIENT
Overall heat transfer coefficients have been calculated according to the following equation:
U Q
TA
Where U is the Overall heat transfer coefficient, T is the temperature driving force and A is the area for heat transfer. In all cases Overall heat transfer coefficients continues to increase with heat load. It can be seen from the tables that the overall heat transfer coefficient is greater at higher steam pressure. Researchers attribute the influence of operating pressure on the overall heat transfer coefficient to a decrease in the reboiler sub- cooled heating zone and subsequent increase in the two-phase evaporating zone (i.e. increase in vapour generation) where overall heat transfer coefficient are greater at higher pressures.
The boiling phenomenon in Chemical industries is preferred because of its excellent heat transfer ability.
Figures 1 and 2 above relate the overall heat transfer coefficient to the temperature difference (driving force). It can be seen from the graphs that the overall heat transfer coefficient decreases linearly as the temperature difference (driving force) increases. We obtained the highest Overall heat transfer coefficient at the smallest temperature difference, which is in correlation with the general principle of a two-phase thermosyphon reboiler.
It can be seen from tables 1 and 2 that the overall heat transfer coefficient increases with increasing heat flux indicating the dominance of nucleate boiling.
As discussed above Overall heat transfer coefficient is strongly dependent on the heat flux and pressure and weakly dependent on Vapour fraction and mass flow, indicating that the nucleate boiling is the dominant mechanism.
-
CONCLUSIONS
In this study, the boiling heat transfer in a thermosyphon apparatus and relationship to the liquid circulation was investigated, several tests were performed and the results compared with the existing results. The conclusions from the present study are as follows.
The temperature distribution along the thermosyphon wall in the evaporator section was almost isothermal. The measured temperature along the condenser showed lower values. This drop of temperature is expected because of the internal resistances due to boiling and condensation.
The rate of circulation is affected by the heat transfer coefficient, hence at higher heat transfer coefficient it is expected to have a higher circulation rate also which fulfils our objective for this study.
Boiling heat transfer is most preferred in chemical industry because it offers a better heat transfer.
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