Essay: Experimental and theoretical Study for convection Heat Transfer in Cylindrical Duct with disk baffle

Abstract: The paper describes an investigation for the thermal design of a fluidized bed cooler and prediction of heat transfer rate among the media categories. It is devoted to the thermal design of such equipment and their application in the industrial fields. It outlines the strategy for the fluidization heat transfer mode and its implementation in industry. The thermal design for fluidized bed cooler is used to furnish a complete design for a fluidized bed cooler of Sodium Bicarbonate. The total thermal load distribution between the air-solid and water-solid along the cooler is calculated according to the thermal equilibrium. The step by step technique was used to accomplish the thermal design of the fluidized bed cooler. It predicts the load, air, solid and water temperature along the trough. The thermal design for fluidized bed cooler revealed to the installation of a heat exchanger consists of (65) horizontal tubes with (33.4) mm diameter and (4) m length inside the bed trough.
Keywords: Fluidization, powder technology, thermal design, heat exchangers
1. Introduction
Many engineering applications with baffles such as thermal regenerator, Shell and tube type heat exchanger, Internal cooling system of gas turbine blades, radiators for space vehicles and automobiles, etc. are applied to enhancement the heat transfer. Baffles are applied as turbulence promoters. Baffles are one method that used to increase the thermal performance. Thermal boundary layer is breaking as fluid flowing through the baffle surfaces.
Most of the baffles found in literature are square, rectangular, triangular, helical or wedge shaped in the present work disk baffles have been studied.
Many techniques have theoretically and experimentally studied the have investigated flow and heat transfer through baffle channel numerically and experimentally. Mayank Vishwakarma et al. (2013) [1] an attempt made to decrease the pressure drop and to increase the heat transfer and the ratio of heat transfer and pressure drop in shell and tube type heat exchanger by tilting the baffle angle up to which we get the minimum pressure drop. It analyzes the conventional segmental baffle heat exchanger using the Kern’s method with fixed shell side flow rates and varied volume flow rate. The results in this method give us clear idea that the ratio of heat transfer coefficient per unit pressure drop is maximum in helical baffle heat exchanger as compared to segmental baffle heat exchanger. Sunil S. Shinde et al. (2012) [2] both hydrodynamic studies & testing of heat transfer & the pressure drop on research facilities & industrial equipment showed much better performance of helically baffled heat exchanger when compared with conventional ones. The new design reduces dead zones within the shell space. These results in relatively high (Heat transfer co-efficient/Pressure drop) & low shell side fouling. Thus, the helix changer exhibits much more effective way of converting pressure drop into a useful heat transfer than conventional heat transfer. This project is basically gives the performance of shell & tube heat exchangers with helical baffles.
Kang-Hoon Ko, N.K. Anand (2003) [3] have investigated experimentally the average heat transfer coefficient in a rectangular channel which was heated from all the four sides, porous baffles were mounted alternately on the top and bottom walls in staggered manner. Reynolds number was varied between 20,000 and 50,000. The experiment was conducted with three different pore densities (viz.: 10 PPI, 20 PPI, and 40 PPI) and two different thickness (viz.: 1and 0.25in.). Material used for baffle was aluminum foam material.
EL-SHAMY (2006) [4] determined the effect of turbulent flow and heat transfer behavior in an annulus with perforated disc-baffles aligned along the inner heated tube surface, using air as a working fluid. The effects of the baffle spacing, the baffle open area ratio, and flow Reynolds number on the thermal performance were examined.
In the present study, experimental and theoretical study the problem of laminar forced convection in a horizontal circular duct with disk baffle. The circular duct is composed of disk baffle and three heaters which all configuration peaks lie in an in-line arrangement. The flow is steady, laminar, incompressible and two dimension. The experiments were conducted to examine the effect of using baffle on heat transfer and fluid flow characteristics of air flow in a tube.
2. Experimental Apparatus:
General
The geometry of the experimental setup is illustrated in Fig. 1. Air flowing into the open tunnel is discharged by blower into the entrance section , a straightened, mixing device, test section, and is passed through orifice then discharged to the atmosphere. a A 5 kW blower flows the air stream and then passed through a calming section duct to damp the turbulence in flow, as shown in Fig. 2. Air passes through the heat transfer test section, orifice meter that measure the flow rate, and then discharged to the atmosphere.
Figure 1: disk baffle in duct
Fig. 2 Schematic diagram of experimental setup
Fig. 3: Test section fitted with disk baffle
2- Experimental Rig:
In this investigation the experimental apparatus is shown schematically in Fig.(1). The experimental rig consists of a blower, entrance duct, a test-section, an orifice flow meter, and instrumentations. The instrumentations are measure temperatures, pressure drop, air flow rate, and electrical power input.
A 7.5 kW centrifugal type air a blower is used to driven air to the circular entrance section. The air is passed to the entrance section with length (1m) and diameter (10 cm) through calming section to damp the turbulence in flow and helps smoothing of pressure fluctuations in the inlet air, as shown in Fig. 1.
2-2 Test section
The air is passed to the test section which is circular duct (10) cm diameter with length (70cm) as shown in figure 3. To minimize the heat loss from test section to the ambient, the duct is insulated with 5 mm thick coated fiberglass standard sheet.
2-3 Baffle:
6 Disk is placed in rod with length (70 cm) which all configuration peaks lie in an in-line arrangement. The disk diameter is (10cm). The distance between one to other is (10 cm). The flow is steady, laminar, incompressible and two dimension. The geometry and coordinate system is shown in Fig. 5.
Fig. 4: Disk baffles
2-4 Test rig instruments
2-4-1 Temperature
The K-type copper-constantan thermocouples with an accuracy of 0.2% of full scale are used to measure air temperature from entrance section as shown in figure 7. Six thermocouples were situated along the test section wall surface to find out the average Nusselt number.
2.4.2 Power Supply
Three flexible electrical wire heaters used for heating the test section provided a uniform heat flux. Heaters are placed on the inner surface of duct with the same distance from on to the other of the perimeter circular duct. The power of each heater is (600 Watt). The electrical output power was controlled by a variac transformer to obtain a constant heat flux along the entire length of the test section.
2.4.3 Air Flow Rate
The air flow rate to the test rig was measured by an orifice designed according to B.S.1042, with (1.23) cm diameter, as shown in Fig. 3. The orifice meter was calibrated using a pitot tube with an accuracy of 2% of full scale are employed to measure the air volumetric.
2.4.4 Pressure
There are two pressure taps on each wall upstream and downstream of the test section and the orifice plate. Carbon tetra-chloride is the working fluid in the U-duct manometer used to measure the pressure drop across the orifice meter with specific gravity (SG) of 1.588.
There are two gauge pressures to measure the pressure drop across the test section.
Fig. 4: Disk baffles in duct
3- Experimental calculation:
In the apparatus setting above, the inlet bulk air at 37.5 C from a blower was directed through the orifice and passed to the heat transfer test section.
For each test run, it was necessary to record the data of temperature at steady state conditions in which the inlet air temperature was maintained at 37.5C.
”””””’ 2
Where :
””””’ 2
Where:
””””’. 3
The convection heat transfer from the test section can be written by:
””””’. 4
Where:
””””’. 5
where Tw is the local wall temperature and evaluated at the outer wall surface of the inner tube.
The average wall temperatures are calculated from 6 points, lined between the inlet and the exit of the test pipe.
The heating surface area, A based on the inner tube diameter (D) was used in all calculations for tube with/without turbulators.
The various characteristics of the flow, the Nusselts number, and the Reynolds numbers were based on the average of tube wall temperature and outlet air temperature.
The local wall temperature, inlet and outlet air temperature, and air flow velocity were measured for heat transfer of the heated tube with a disk tape. The average Nusselt numbers were calculated and discussed where all fluid properties were determined at the overall bulk mean temperature. Then we can calculate Nusselt number:
”””””’ 6
Where:
= duct diameter
The Reynolds number is given by:
””””.. 7
5. Results:
In this work the analysis was done for inserting baffles into duct that devices promote mixing of coolants and baffles can significantly disturb the air flow.
6. Conclusion:
NOMENCLATURE
A Area, (m2)
Air cooling load, (kW)
Ac Cross section area, (m2)
Total cooling load, (kW)
Ar Archimedes number Re Reynolds number (Dimensionless)
CD Drag coefficient T Temperature, (??C)
Cp Specific heat, (kJ/ Kg.C) ??pb Bed pressure difference, (N/m??)
dp Particle diameter , (m) ??pd Distributor pressure difference, (N/m??)
Do Outside tube diameter, (m) ??T Temperature difference, (deg C)
Fluidized bed porosity Umf Minimum fluidization velocity, (m/s)
g Gravitational acceleration , (m/s2) uo Gas velocity, (m/s)
h Heat transfer coefficient, (W/m2 K) Uo Overall heat transfer coefficient, (W/m2 K)
Minimum fluidization bed height, (m) Uor Velocity through orifice distributor, (m/s)
k Thermal conductivity, (W/m K) Ut Terminal particle velocity, (m/s)
L Trough length, (m)
LMTD Logarithmic mean temperature difference
Solid mass, (kg)
Mass flow rate, (kg/s)
Nt Total Number of tube
Nu Nusselt number, Dimensionless
Pr Prandtl number (Dimensionless)
Subscript
a Air
g Fluidizing gas
i Initial value
mf Minimum fluidization condition
o Outlet value
p particle
s Solid
w Water
Greek Symbols
‘ Difference
?? Fluid viscosity, (Pa.s)
?? Fluid density, (kg/m3)
Sphericity
References
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