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Abstract
In this research, experiments are performed to study the laminar flow mixed-convection heat transfer with radiation effects for a hydrodynamically fully developed and thermally developing airflow in a horizontal duct. The duct cross section is made of two differentially heated isothermal vertical walls and two adiabatic horizontal walls with aspect ratios 1 and 0.5. The total heat transfer from the hot wall to the cold wall of the duct depends on the mixed convection and also on the surface radiation heat transfer that takes place within the duct. The analysis of experimental data for the Nusselt number from the hot wall of the duct shows that it is important to consider the effects of surface radiation, and to accounted for them, in the design and analysis of flow and heat transfer through ducts. The flow field within the duct is also made visible by a suitable smoke flow visualization method. The heated air moves upward, accumulates near the top wall adjacent to the hot wall of the duct, and gets circulated continuously to the cold wall on the opposite side. The accumulated flow is thermally stable; the stable conditions can reduce the heat transfer enhancement due to the buoyancy force. The results show that flow condition and surface radiation interaction significantly affect the total Nusselt number and the surface temperature variation along the walls.
NOMENCLATURE
| AR | = | duct aspect ratio, W/H |
| As | = | area of hot wall surface, m2 |
| Dh | = | hydraulic diameter, Dh = 2(W H) / (W+H) |
| Gz | = | Graetz number |
| H | = | height of the duct, m |
| h | = | heat transfer coefficient, W/m2-K |
| k | = | thermal conductivity of air, W/m-K |
| L | = | length of the test section, m |
| Le | = | length of the entrance section, m |
| Nu | = | Nusselt number, Nu = h Dh/k |
| Pr | = | Prandtl number |
| q | = | heat flux, W/m2 |
| Q | = | heat flow, W |
| Ra | = | Rayleigh number |
| Re | = | Reynolds number, Re = Uin Dh/ν |
| Ri | = | Richardson number |
| Th | = | hot wall temperature, K |
| Tin | = | air entry temperature to the test section, K |
| Tm | = | mean temperature (Tin + Tout)/2, K |
| Tout | = | air exit temperature from the test section, K |
| W | = | width of the duct, m |
| Z | = | dimensionless z-coordinate, Z = z/Dh |
| z | = | distance from inlet of the test section, m |
Greek Symbols
| ϵ | = | emissivity |
| ν | = | kinematic viscosity, m2/s |
Subscripts
| conv | = | convection |
| cond | = | conduction |
| in | = | inlet |
| out | = | outlet |
| rad | = | radiation |
| tot | = | total |
Additional information
Funding
Notes on contributors
Rajamohan Ganesan
Rajamohan Ganesan is a senior lecturer in mechanical engineering at Curtin University, Sarawak, Malaysia, and he completed his Ph.D. in Mechanical Engineering with Curtin University, Perth, Australia. He received his M.Eng. and B.Eng. degrees from the Bharathidasan University, India. Prior to joining Curtin Sarawak campus, he worked as a lecturer in the Department of Mechanical Engineering, TAFE College, Malaysia, for three years. He is currently working on mixed convection heat transfer in ducts.
Ramesh Narayanaswamy
Ramesh Narayanaswamy obtained his Ph.D. degree from the Indian Institute of Technology, Madras, India. He is currently a senior lecturer in the Department of Mechanical Engineering at Curtin University, Australia. His research interests address basic and applied problems in heat transfer.
Kumar Perumal
Kumar Perumal obtained his Ph.D. degree from the University Institute of Chemical Technology, Mumbai, India. He is currently an associate professor in Curtin University, Sarawak Campus Malaysia. Over the years, he has been working in the area of mixing in stirred vessels, static mixing, process intensification, and circulating fluidization.