# An experimental investigation of heat transfer in forced convective boiling of R134a, R123 and R134a/R123 in a horizontal tube

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## Abstract

This paper reports an experimental study on flow boiling of pure refrigerants R134a and R123 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10 mm located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6 MPa in the heat flux ranges of 5–50 kW/m^{2}, vapor quality 0–100 percent and mass velocity of 150–600 kg/m^{2}s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen’s superposition model, a new correlation is presented for heat transfer coefficients of mixture.

## Key Words

Convective Boiling Flow Pattern Heat Transfer Horizontal Tube Mixture## Nomenclature

*a*Thermal diffusivity (m

^{2}/s)*Bo*Boiling number (=q/Gh

_{fg})*Co*Convection number ( = (ρ

_{υ}/ρ_{ι})^{0.5}( (1-β/β^{0.8})*C*_{p}Specific heat (J/kgk)

*D*Tube Diameter (m)

*g*Gravitational acceleration (m/s

^{2})*h*Specific enthalpy (J/kg)

*h*_{fg}Latent heat of vaporization (J/kg)

*Fr*Froude number

*G*Mass velocity (kg/m

^{2}s)*k*Thermal conductivity (W/mK)

*P*Pressure (Pa)

*q*Heat flux (W/m

^{2})*Re*Reynolds number

*T*Temperature (K)

*X*Mole fraction in liquid

*X*_{tt}Martinelli parameter

*Y*Mole fraction in vapor

*z*Axial distance (m)

## Greek Letters

- α
Heat transfer coefficient (W/m

^{2}K)- β
Vapor quality

- θ
Contact angle (=35°)

- ΔT
_{bp} Boiling point range (K)

- ΔT
_{id} Ideal wall superheat (K)

- ρ
Density (kg/m

^{3})- σ
Surface tension (N/m)

- μ
Viscosity (Pa · s)

- ν
Kinematic viscosity (m

^{2}/s)

## Subscripts

*c*Critical

*in*Inlet of the heat transfer section

*l*Liquid

*nb*Nucleate boiling

*s*Saturation

*v*Vapor

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