Experimental analysis of conical baffles with rifts on heat transfer and pressure drop

ABSTRACT

In the present study, convective heat transfer to the air from a heating tube attached to conical baffles with rift was experimentally examined. The air entering the test section first contacts the large surface of the conical baffle. Therefore, the conical baffle both directs the air toward the heating surface and increases the heat transfer surface area. In the experiments, baffles with inclination angles of 45°, 60°, and 80° were used. The baffles were placed on the heating tube at the pitch of 15 mm. The temperature of the heating fluid (water) was kept fixed at 65°C. In addition to the riftless baffles, the experiments were carried out by using baffles with a rift spacing of 1.5 and 3.5 mm so that the boundary layer separation mechanism could be accelerated. Experimental results for eight different velocities of airflow (2–20 m/s) were presented. For the inclination angle of 60°, the increase in the heat transfer of the baffle with rift was 13% at a rift spacing of 1.5 mm and 4% at a rift spacing of 3.5 mm according to the riftless baffle. In addition, for the inclination angle of 60°, the pressure drop values of the riftless and the rift spacing of 1.5 and 3.5 mm were almost the same.

KEYWORDS:

• Rift
• conical baffle
• heat transfer
• pressure drop

Nomenclature

${\mathrm{A}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}$	=	Total heat transfer surface area (m2)
${\mathrm{A}}_{\mathrm{s}}$	=	Surface area of tube between two baffles (m2)
${\mathrm{A}}_{\mathrm{b}\mathrm{a}\mathrm{f}\mathrm{f}\mathrm{l}\mathrm{e}}$	=	Area of the conical baffle on the tube (m2)
${\mathrm{A}}_{\mathrm{p}}$	=	Area of section vertical to the direction of flow between two baffles (m2)
${\mathrm{c}}_{\mathrm{p},\mathrm{a}\mathrm{i}\mathrm{r}}$	=	Specific heat of air (kJ/kg °C)
${\mathrm{c}}_{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}$	=	Specific heat of water (kJ/kg °C)
c	=	Coefficient of formulation
D	=	Outer diameter of heating tube (m)
H	=	Height of conical baffle (m)
h	=	Average heat convective coefficient (W/m2 K)
k	=	Thermal conductivity (W/m °C)
L	=	Length of heating tube (m)
${\dot{\mathrm{m}}}_{\mathrm{a}\mathrm{i}\mathrm{r}}$	=	Mass flow of air (kg/s)
${\dot{\mathrm{m}}}_{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}$	=	Mass flow of water (kg/s)
m	=	Coefficient of formulation
n	=	Number of conical baffles
Nu	=	Average Nusselt number
Pr	=	Prandtl number
p	=	Pitch between conical baffles (m)
$\dot{\mathrm{Q}}$	=	Heat power transferred from hot fluid (W)
${\dot{\mathrm{Q}}}_{\mathrm{a}\mathrm{i}\mathrm{r}}$	=	Heat transfer of air (W)
${\dot{\mathrm{Q}}}_{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}$	=	Heat transfer of water (W)
${\dot{\mathrm{Q}}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}}$	=	Convective heat transfer (W)
Re	=	Reynolds number
r	=	Rift spacing (mm)
R2	=	Coefficient of determination
${\mathrm{T}}_{\mathrm{i},\mathrm{a}\mathrm{i}\mathrm{r}}$	=	Inlet temperature of air into test section (°C)
${\mathrm{T}}_{\mathrm{o},\mathrm{a}\mathrm{i}\mathrm{r}}$	=	Exit temperature of air from test section (°C)
${\mathrm{T}}_{\mathrm{i},\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}$	=	Inlet temperature of water to test section (°C)
${\mathrm{T}}_{\mathrm{o},\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}$	=	Exit temperature of water from test section (°C)
${\mathrm{T}}_{\mathrm{s}}$	=	Temperature of heating tube surface (°C)
${\mathrm{T}}_{\mathrm{\infty }}$	=	Temperature of heated air (°C)
t	=	Conical baffle thickness (m)
Vmax	=	Maximum velocity (velocity between two conical baffles) (m/s)
Vair	=	Inlet velocity of air into test section (m/s)

Greek Symbols

ν	=	Kinematic viscosity (m2/s)
$\alpha$	=	Conical baffle inclination angle ()
${ }_{\mathrm{b}\mathrm{a}\mathrm{f}\mathrm{f}\mathrm{l}\mathrm{e}}$	=	Conical fin efficiency
${\rho }_{\mathrm{a}\mathrm{i}\mathrm{r}}$	=	Density of air (kg/m3)
$\mathrm{\Delta }p$	=	Pressure drop (mbar)

Subscripts

air	=	Air side
conv	=	Convective
i	=	Inlet
o	=	Exit
s	=	Tube wall
total	=	Total
water	=	Water side