title: Two dimensional analysis of low pressure flows in the annulus region between two concentric cylinders.
abstract: A numerical simulation of the steady two-dimensional laminar natural convection heat transfer for the gaseous low-pressure flows in the annulus region between two concentric horizontal cylinders is carried out. This type of flow occurs in "evacuated" solar collectors and in the receivers of the solar parabolic trough collectors. A finite volume code is used to solve the coupled set of governing equations. Boussinesq approximation is utilized to model the buoyancy effect. A correlation for the thermal conductivity ratio (k r = k eff/k) in terms of Knudsen number and the modified Rayleigh number is proposed for Prandtl number (Pr = 0.701). It is found that as Knudsen number increases then the thermal conductivity ratio decreases for a given Rayleigh number. Also, it is shown that the thermal conductivity ratio k r increases as Rayleigh number increases. It appears that there is no consistent trend for varying the dimensionless gap spacing between the inner and the outer cylinder ([Formula: see text]) on the thermal conductivity ratio (k r) for the considered spacing range.
authors.affiliation: Mechatronics Engineering Department, German Jordanian University, Amman, Jordan.
authors.forename: Wael
authors.initials: W
authors.lastname: Al-Kouz
authors.affiliation: Mechanical and Maintenance Engineering Department, German Jordanian University, Amman, Jordan.
authors.forename: Aiman
authors.initials: A
authors.lastname: Alshare
authors.affiliation: Energy Engineering Department, German Jordanian University, Amman, Jordan.
authors.forename: Ammar
authors.initials: A
authors.lastname: Alkhalidi
authors.affiliation: Mechanical Engineering Department, Jordan University of Science and Technology, Irbid, Jordan.
authors.forename: Suhil
authors.initials: S
authors.lastname: Kiwan
keywords: Concentric cylinders; Heat transfer; Low pressure; Natural convection; Parabolic trough; Solar collectors