Visualization of Pool Boiling from Stacked Enhancement Structures

Camil-Daniel Ghiu

Background

The mechanism of boiling from enhanced structures is still not well understood. A few investigators have developed semi-empirical mathematical models to explain the boiling phenomenon from these surfaces and validate the enhancement observed:

• Nakayama et al. (1980)

Developed expressions for evaluating the boiling parameters for the suction - evaporation mode. It was the first model to explain the intermittent nature of boiling from enhanced structures.

• Xin and Chao (1985)

Developed expressions for fully developed boiling (characterized by a steady vapor stem at the pores)

• Webb and Haider (1992)

Developed expressions for the flooded mode.

• Haider and Webb (1996)

Developed expressions for the enhanced convection from the external surfaces of enhanced structures due to intermittent motion of the bubbles. Applicable for a wall superheat range 0-4 ^oC.

• Chien (1996)

Improved Nakayama et al.’s model and reduced the empiricism in the model. Model demonstrated for a wall superheat range 0 - 4 ^oC only.

Motivation for present study

• The existing models do not apply well for boiling from structured surfaces in confined spaces.

• The evaporation process inside the channels was hypothesized based on the observations on the external surface. Visualizing the evaporation process inside the channels is required to validate the assumptions.

• The  transition periods between the boiling flow regimes are not well understood and quantified.

Objectives of the present study

• Establish the mechanism of pool boiling on enhanced structures.

• Based on this mechanism, develop a semi-analytical model for nucleate boiling regime.