HEAT TRANSFER CHARACTERISTICS
5.3. Experimental
The experimental system for measuring the convective heat transfer coefficient is shown schematically in Figure 1.
Chapter-5
138 Table 5.1: Summary of heat transfer studies in helical coil.
Author Parameters Working Fluid Correlation Remarks
Ali et al. (1968) Lt = 10ft.
di = 0.492 do = 625
Dc = 9.86, 20.5in.
Water feed rate = 77-306 lb/h
Heat flux = 19000- 81000 Btu/h ft2
Air and Water
TPF
ggtt dP dL
dL dP
/
/
g refers to gas, ϕstt = Lockhart- Martinelli parameter at turbulent – turbulent condition.
Studied pressure drop and heat transfer and modified Lockhart- Martinelli correlation.
Departure from nucleate boiling was not studied
Rajasekharan et al.
(1970)
Di/Dc = 0.031-0.222 di = 0.6407-0.27cm n = 4.5-12.
Water,
CMC: 0.5 - 1%.
sodium silicate : 1%
7 . 0 7 . 0
4 1 8 3
. 1 98 .
1 GN
n
c i
Nu N
n n D
N D
Studied with both
Pseudoplastic and
Dilatant(Non-Newtonian fluids)
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Heat Transfer Characteristics Janssen and
Hoogendoorn (1978)
di = 5x10-3 m
Di = 5x10-1-6.2x10-2 m L = 4.5-5.9 m
d/D = 1x10-2-8.3x10-2
m
Water glycerol
mixtures. log
0 1
log RePr
4 1
T T T L Nu d
It has appeared that
only for low values of the Dean number (De < 20)
Sethumadhavan and Rao (1983)
di = 2, 3 mm
= 30-750
p = 10,22,38 and 66 L = 1500mm
Water and
glycerol G(h,Pr)tan0.18(Pr0.55)8.6(h)0.13
Given a correlation for heat transfer.
Austen and Soliman (1988)
di = 4.37 do = 6.34
Dc/d = 29.1-56 h/d = 5,58,60 D/d = 29-532
Water fC/ fS 10.033(logDnm)4 Done a pressure drop and heat transfer coefficient
Chapter-5
140 Ghorbani et al.
(2010)
di = 7.77,10.82 mm; do = 9.47, 12.59 mm; Di = 125.71,128.31 mm; p = 16.47, 23.57 mm; N = 23.25, 16.25.
water NuDeq 0.0041Ra0Deq.4533Re0Deq.2 Prs0.3 Studied mixed convection heat transfer in helical coiled tube heat exchanger
Pawar and
Sunnapwar (2012)
di = 15.6 mm; Di = (320, 280, 260, 220) mm; L = (2612, 2813, 3593, 3920, 3619) mm; N = 4.4, 4.2, 3.9, 3.6, 2.6
Oil & Nano fluids (weight
concentrations of 0.1, 0.2 & 0.4%)
3 / 1 3 /
]1
4 / ) 1 3 [(
75 .
1 n n Gz
Nu
where Gz is the Graetz number
Showed that overall heat transfer coefficient is higher for smaller helix diameter as compared to larger helix diameter due to significant effect of centrifugal force on secondary flow in coil
TH-1484_10610718
Heat Transfer Characteristics
OUTLET
v
PUMP
TANK
COMPRESSOR
R1 R2
R1 – Rotameter for water R2 – Rotameter for air V1 - V6 - Valves
T2
T1
T1 – Thermocouple at inlet T2 – Thermocouple at outlet S 1– Temperature sensor
D1 – Data Requisition system P1 & P2 – AC Power supply
Temperature controller
S1
D1 1
P1
Thermal insulation sheet Copper tube
Cooling system
P2
V1
V2 V3 V4
V5
V6
Figure 5.1: Schematic diagram of experimental set up.
It consists of a flow loop, a heating unit, a measuring unit and a control unit. The flow loop included a pump, two rotameters, a reservoir, and a test section. Three helical copper tube with 3500 mm length were used as the test section. Table 5.2 summarizes the geometrical specifications of the three coils tested in the present study. The whole test section was heated by a tape heater which is linked to an AC power supply. The maximum heating capacity of the tape heater was 500 W. There was a thick thermal insulating layer surrounding the heater to obtain a constant heat flux condition along the test section. Two K-type thermocouples were mounted on the test section, one at the inlet and other at the outlet of the helical coil.
Chapter-5
142
The temperature readings from the thermocouples were registered by a digital data requisition system. The experiment was carried out for air-water system as well as air-SCMC system.
Table 5.2: Geometric specification of the different coils tested
Coil Dc (m) dt (m) N
Coil 1 0.2 0.01 4.5
Coil 2 0.15 0.01 6
Coil 3 0.2 0.015 4.5
The concentration of SCMC (Sodium Carboxy Methyl Cellulose) used were 1.0, 1.5 and 2.0 kg/m3. The physical properties of water and SCMC solutions are shown in Table 5.3.
Table 5.3: Physical Properties of the SCMC solution Fluid Concentration
kg/m3
Flow behaviour
index, n
Consistency index
k,
Density, l, kg/m3
Surface tension, l,
N/m
SCMC-1 1.0 0.9099 0.025516 1000.83 0.0721
SCMC-2 1.5 0.8701 0.036167 1001.69 0.0727
SCMC-3 2.0 0.8338 0.051265 1001.96 0.0732
Water - * * 999.70 0.0710
* Viscosity of water : 0.79710-3 kg/m.s
** Viscosity of air : 1.86310-5 kg/m.s
For the specific heat capacity, the calculations of Semmar et al. (2004) has been referred, where they have calculated the specific heat of SCMC solution at different SCMC concentrations. The correlation that has been used to calculate the specific heat is given by
T A
T T
B
T T
C CP 0 2 0
(5.4)
TH-1484_10610718
Heat Transfer Characteristics where, T0 = 273.150, K and T is the wall temperature. The values of polynomial coefficients used in calculating the specific heat using Eq. (5.4) is shown in Table 5.4. Using these values a calibration curve is drawn from where a polynomial equation was developed which is related to specific heat (Cp) capacity with SCMC concentration.
Table 5.4: Values of polynomial coefficients at different SCMC concentrations
Coefficients Pure water SCMC (18 K/m3) SCMC (35 K/m3) SCMC (83 K/m3)
A (Jkg−1 K−1) 0.015 -0.032 -0.015 0.0317
B (Jkg−1 K−1) -1.145 6.953 5.441 -2.366
C (Jkg−1 K−1) 4215.6 4061.6 4231.2 4592
Thus a polynomial equation is obtained by fitting the experimental data of Semmar et al.
(2004) can be represented by
1895 . 4 0099
. 0 10
7 5 2
SCMC SCMC
P C C
C (5.5)
By putting the values of different SCMC concentration used in the experiment, Cp values of the solution were calculated from the fitted Eq. (5.5). The correlation coefficient for goodness of fit of Eq. (5.5) was found to be 0.9939. The thermal conductivity of the SCMC solution was calculated by the correlation developed from the experimental data of Carezzato et al.
(2007). The correlation that has been developed can be expressed as 643
. 0 000
. 0 10
4 5 2
CSCMC CSCMC
(5.6)where ‘κ’ denotes thermal conductivity and ‘CSCMC’ denotes SCMC concentration. By putting the values of different SCMC concentrations used in the experiment, the corresponding thermal conductivity values of the solution were calculated from the correlation. The uncertainty of the experimental data is shown in Table 5.5.
Chapter-5
144
Table 5.5: Range of the Means, Standard Deviations, and Uncertainties of the heat transfer coefficient at constant tube diameter (dt = 0.015 m), coil diameter (Dc = 0.20 m)
Usl (m/s) Usg
(m/s)
SCMC (kg/m3)
Range of Mean*
(w/m2k) N
Range of STDEV*
(× 10-3)
Range of uncertainty*
(× 10-3)
Range of relative uncertainty
% 0.032-0.079 0.019 1.0 0.243-0.388 6.0 4.852-5.562 1.982-2.274 0.515-0.937 0.032-0.079 0.038 1.0 0.265-0.426 6.0 3.691-5.781 1.513-2.363 0.557-0.571 0.032-0.079 0.057 1.0 0.297-0.468 6.0 4.854-5.051 1.981-2.061 0.424-0.697 0.032-0.079 0.076 1.0 0.315-0.502 6.0 4.793-4.864 1.966-1.983 0.391-0.624 0.032-0.079 0.019 1.5 0.205-0.326 6.0 3.264-4.353 1.332-1.782 0.407-0.862 0.032-0.079 0.019 2.0 0.186-0.267 6.0 4.042-4.275 1.651-1.745 0.655-0.893
* The theory to calculate the values is given in chapter 2 in equations (2.10 – 2.12)
Each value of heat transfer coefficient in 4th column corresponds to the mean of 6 data points (i.e., N=6). Relative uncertainty is calculated from standard uncertainty and mean value of the corresponding data set.