**5. Example**

Determine the heat transfer coefficient and pressure drop of refrigerant R1234ze during boiling process inside horizontal microfin tube at 5°C and heat flux 8,62 kW/m<sup>2</sup> , mass flux 222 kg/m<sup>2</sup> s. The geometry of microfin tube is 60 number of fins, the inner surface diameter is 8.96 mm, fin height is 0.2 mm, the helix angle and apex angle are 18° and 40°, respectively.

Solve:

At saturation temperature tsat = 5°C of refrigerant R1234ze, thermal properties could be taken in **Table 1**.

Refrigerant: R1234ze, molar mass: M = 114 g/mol, pcritical = 3.64 MPa

Reduce pressure: pr = psat/pcritical = 0.2593/3.64 = 0.07124

Heat flux: q = 8.62 kW/m<sup>2</sup>

Mass flux: G = 222 kg/m<sup>2</sup> s

Microfin tube: N = 60 number of fins, dr = 8.96 mm, e = 0.2 mm, β = 18o; γ = 40°


**Table 1.** *Properties of R1234ze.*

## **5.1 Flow pattern map**

Apply the flow pattern map of Rollmann and Spindler [1] to present in here. At first, calculate the transition lines between one regime to another as a function of quality x and use the classification of the regimes on the map:


In this case, the flow pattern map for boiling of R1234ze in microfin tube was built at 5 °C saturation temperature, heat flux q = 8.62 kW/m<sup>2</sup> , with the fixed mass flux G = 222 kg/m<sup>2</sup> s.

The result map shown in **Figure 6**, when the mass flux lower than 50 kg/m<sup>2</sup> s, the flow boiling during the changing phase just only occur at the fully stratified flow. But if working condition at mass flux 200 kg/m<sup>2</sup> s, at the beginning of boiling process, the quality is still low, the boiling occur at slug/helix flow until the quality reach to 0.15, the helix flow happen, and keep boiling with the helix flow until quality is 0.25, it moves to the annular flow boiling to the rest of changing phase.

**Figure 6.** *Flow pattern map on microfin tube.*

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