P.K. Sarma

Laminar convective heat transfer with twisted tape inserts in a tube

Abstract A new method is postulated to predict heat transfer coefficients with twisted tape inserts in a tube, in which the wall shear and the temperature gradients are properly modified through friction coefficient correlation leading to heat transfer augmentation from the tube wall. The eddy diffusivity expression of van Driest is modified to respond to the case of the internal flows in a tube with twisted tapes for ranges of Reynolds number corresponding to the laminar flow in tubes. The predictions from the present theory are compared with some correlations available in literature for twisted tape inserts. The present theoretical results are rendered in the form of a correlation equation.

The effects of thermal non-equilibrium and inlet temperature on two-phase flow pressure drop type instabilities in an upflow boiling system

Abstract In this paper a theoretical model for the two-phase flow pressure drop type instabilities in an upflow boiling system is presented. The thermal non-equilibrium effect between the two phases is included assuming the enthalpy profile in the subcooled boiling region. The system of differential equations describing the single-phase and boiling regions of the system (drift-flux model) is solved using finite difference method for the steady state characteristics of the system over a wide range of operating conditions. Upon obtaining the steady state characteristics, the dynamic formulation of the pressure drop type oscillation is solved numerically. The modeling results are verified by the experimental findings. The effect of the thermal non-equilibrium on the steady state characteristics, stability boundaries and oscillation periods at different heat inputs and inlet temperatures are presented as being compared with the experimental measurements as well as the results obtained from the thermal equilibrium model.

Convective condensation heat transfer in a horizontal condenser tube

The purpose of this article is to solve analytically the problem of convective condensation of vapors inside a horizontal condenser tube. Homogeneous model approach is employed in the estimation of shear velocity, which is subsequently, made use of in predicting local convective condensation heat transfer coefficients. The resulting analysis of the present study is compared with some of the available equations in the literature. It is observed that the agreement is reasonably satisfactory validating the assumptions and the theory presented.

Condensation of vapours on a fin in the presence of non-condensable gas

Abstract Condensation of vapours on a vertical fin in the presence of non-condensable gas is formulated and solved for a wide range of system parameters, making use of the analogy between heat and mass transfer. The formulation enables the derivation of special cases such as condensation of vapours on a vertical isothermal surface with and without non-condensable gas in the vapour. Besides, the analysis of condensation of vapour on a fin has revealed non-isothermal conditions of the thin fin with the temperature of the liquid–vapour interface also changing along the length of the fin. The numerical results from the analysis are rendered into dimensionless equations that can be used to predict the mean condensation heat transfer coefficients.

A method to predict two-phase pressure drop using condensation heat transfer data

Abstract The data of Kutateladze related to mean condensation heat transfer coefficients of pure vapors of steam flowing in a horizontal tube are correlated considering the phenomenon as a homogenous flow. Colburns analogy assuming to hold good the mean friction coefficients for a homogenous flow conditions are evaluated making use of heat transfer data. By extending the model further the two-phase frictional pressure drop multipliers are established. Predictions from these correlations have reasonably agreed with the magnitudes obtained from Lockhart and Martinelli correlations which are conventionally used in the design.