**1. Introduction**

The boiling process of Refrigerant is very important on the designing of evaporator on the refrigeration system. The optimization of heat exchanger about the size, weight and heat transfer performance is the main problem. In order to solve this problem, microfin tube is the ones should pay attention.

To understand the boiling process, the pattern of flow should be known during the boiling of refrigerant. With the type of flow patterns and the general map of flow pattern for boiling in smooth tube is very clear it is concluded fully stratified, slug flow, stratified wavy flow, intermittent flow, annular flow, dryout and mist flow. While for microfin is still unclear especially for the newest refrigerant. With the method of Rollmann et al. [1], the pattern of boiling in microfin tube could be classify in such fully stratified flow, stratified wavy flow, the combination of slug and stratified wavy flow, helix flow, annular flow. In order to build this method, Rollmann et al. was updated the method of Wojtan et al. [2] which was introduced for smooth tube. Those method based on the mathematical model of two phase stratified the geometries of flow from [3] to determine the geometries of flow. Besides that, in recent years some new method to build the map for flow pattern are still updated from Zhuang et al. [4] and Yang et al. [5].

In case of internal flow, the main difficulty, concerning to the evaluation of the heat transfer coefficient and friction factor, is related to the flow regimes (distribution of liquid and vapor in a cross-section). As it is easily understood, the main heat transfer mechanism changes if, at the wall, there is liquid, vapor, both of them, droplet impingement, bubble formation and so on. Every flow regime requires a specific analytical description and appropriate criteria to state if it occurs or not. As quality, liquid mass flux and vapor mass flux change along a duct, several flow regime onsets along a duct in the presence of heat transfer. The criteria are usually represented in two-dimensional diagrams called flow pattern maps. The internal geometry of the enhanced tubes and the thermal properties of the fluid affect the distribution of the phases in the cross section.

It is obviously that the outstanding performance of heat transfer in microfin tubes for boiling is valuable to consider without introducing excessive penalization in the pressure drop. Those have been proven in so many experimental researches which are published in recent years and also presented in the paper of Phan et al. [6]. Besides, with the correlations for heat transfer of evaporation process on microfin tube are published would be introduced in this chapter are come from Thome et al. [7], Cavallini et al. [8], Yu et al. [9], Yun et al. [10], Chamra et al. [11], Wu et al. [12], Rollmann and Spindle [13].

Also for the pressure drop of two phase flow, some correlations were built for both evaporation and condensation Choi et al. [14], Goto et al. [15], the others used for separated purposed. Some particular pressure drop correlations could be mentioned for evaporation Rollmann and Spindler [13], Wongsa-Ngam et al. [16] and Kuo et al. [17].

About this problem, Phan Thanh Nhan [18] focused on the experimental test to determine flow pattern, heat transfer and pressure drop for both boiling and condensation in microfin tube for two kind of Refrigerants R134a and R1234ze.

This chapter is demonstrated in convective boiling of refrigerant in horizontal microfin tube. Flow pattern map, heat transfer coefficient and pressure drop during boiling process would be presented with the published correlations and taken on working condition for boiling as an example to calculate step by step.

#### **2. Flow pattern map**

In order to draw a map of flow mechanism, there needs to be a classification of different regimes of flow in microfin tube, which relies on flow patterns such as slug, fully stratified, stratified wavy, helix, annular and some regimes are the combination of two or three flow pattern simultaneously happened. To build the map, many parameters have been cleared about geometries of tube, heat flux, mass flux, thermo-physical and thermodynamics properties of fluid, local quality, quality change of boiling processes and define the void fraction, Martineli parameter or also estimate the shape of flow (about the liquid part and vapor part consisted on the position of tube).

Even many different group researches, they also classified the map into some main regimes: bubbly flow, plug flow, slug flow, intermittent flow, stratified flow, stratified wavy flow, annular flow, dry-out regime, mist flow or transition regime, other differences just only the difference name they called for the same regime. Flow patterns in horizontal flows are illustrated in **Figure 1**. Depend on every single group they have every different name for their classification regimes. Every regime can be described as below:

*A Review on Convective Boiling Heat Transfer of Refrigerants in Horizontal Microfin-Tubes… DOI: http://dx.doi.org/10.5772/intechopen.100024*


For micro-fin tube, those the last few years, some other detailed regimes are called for flow patterns:


**Figure 1.** *Flow regime for boiling from Collier and Thome 1994 [7].*
