Determination of the operation range of a vertical two-phase closed thermosyphon

Abstract

A comprehensive model, proposed for a vertical two-phase closed thermosyphon (TPCT) by the present authors, is further developed by utilizing the criteria for dryout, flooding and boiling limits to investigate the effects of filling ratio on them together, while the available models can just consider one or two limits of them. A new concept named dryout ratio is proposed, which can be used for predicting dryout limit. The empirical correlation and the empirical value, provided by other researchers, are used for predicting flooding and boiling limit, respectively. The experiments with nitrogen as working fluid are performed, and compared with the calculations. The maximum filling ratio is introduced, beyond which the liquid could be carried to condenser and the heat transfer performance can be deteriorated. And then the closed operation range of a vertical TPCT is finally determined, which has not been reported before. The effects of operating pressure and geometries on the range are also analyzed. List of symbolsHeat transfer rate (W) qRadial heat flux (W m -2 ) ReReynolds number T Temperature (K) uFlow velocity (m s -1 ) V vj Vapor drift velocity (m s -1 ) xAxial coordinate y Radial coordinateGreek symbols d Liquid film thickness (m) r Surface tension (N m -1 ) q Density (kg m -3 ) a Void fraction l Dynamic viscosity (Pa s) m Kinematic viscosity (m 2 s -1 ) s Shear stress (N m -2 ) k Thermal conductivity (W m -1 K -1 ) C Mass flow rate per unit width (Kg m -1 s -1 )

Two-phase closed thermosyphons: A review of studies and solar applications

“…The minimum mass of working fluid is required for the continuous circulation of vapor and liquid film. When the available fluid mass is smaller than the minimum for a given heat flux, the bottom of evaporator experiences dryout, which can cause dry spots to form on the heated surface of the walls of the evaporator [7,50,51].…”

Unsteady experimental and numerical analysis of a two-phase closed thermosyphon at different filling ratios

“…When filling ratio is more than optimum one, the liquid film and liquid pool are continuous. However, when filling ratio is less than optimum, a film dryout in the evaporator could occur [19,31]. It is considered in the numerical simulation that dryout limit is reached when the liquid pool is completely dried out (L p =0).…”

“…Harley and Faghri [18] in 1994 presented a model that coupled a general quasi-steady Nusselt type solution of the falling film with the complete two-dimensional vapor solution in order to simulate the transient behavior of TPCT without considering liquid pool. Jiao et al [19] developed a new steady state model, based on experimental previous observations. They affirm that three types of flow pattern and two types of transition can be observed, according to the distribution of liquid film and liquid pool can be observed during the thermosyphon operations.…”

Heat Pipes: Review, Opportunities and Challenges

“…The effects of heat input, operating pressure, and geometries of the thermosyphon on the range of the filling ratio were also discussed. Jiao et al (2012) further developed the model presented by Jiao et al (2008) to investigate the effect of the filling ratio on dryout, flooding and boiling limits. The experiments were conducted using nitrogen as working fluid and the experimental results were compared with the calculations.…”