Flow Characteristics of R32/R134a Falling-Film Evaporation Outside a Horizontal Tube

Research Overview

This study numerically investigates falling-film evaporation of R32/R134a non-azeotropic refrigerant outside a horizontal tube, focusing on flow characteristics and liquid film thickness distribution. A two-dimensional VOF model was established by incorporating multicomponent phase change and contact angle models into the governing equations. The effects of spray height, tube diameter, Reynolds number, inlet temperature, and R32 mass fraction in the liquid phase on local and average liquid film thickness were analyzed.

Graphical Abstract

Figure: One-page graphical summary of a horizontal-tube falling-film evaporator, R32/R134a non-azeotropic refrigerant, two-dimensional VOF numerical model, multicomponent phase change, liquid film thickness distribution, tangential velocity, wall shear stress, minimum liquid-film-thickness location, and engineering significance for evaporator design.

Background and Objective

Horizontal-tube falling-film evaporators are regarded as effective alternatives to flooded evaporators because they can provide higher heat transfer coefficients and lower refrigerant charge.

In falling-film evaporation, liquid film thickness and its circumferential distribution strongly influence thermal resistance, flow stability, local dryout, and evaporator performance. Accurate prediction of local film thickness is essential for preventing heat transfer deterioration caused by dry patches.

For non-azeotropic refrigerants, temperature and concentration gradients appear during evaporation, and thermophysical properties vary nonlinearly with temperature and composition. Therefore, knowledge from water or pure refrigerants cannot fully describe R32/R134a film flow.

The objective of this study is to clarify the flow characteristics and film thickness distribution of R32/R134a falling-film evaporation outside a horizontal tube and to provide design guidance for high-efficiency, low-charge evaporators.

Key Features of This Study

Non-azeotropic refrigerant FFE: R32/R134a mixture film flow outside a horizontal tube was investigated.
Multicomponent phase-change model: Component diffusion, interfacial evaporation, latent heat, and thermophysical mixing rules were included.
VOF interface tracking: The gas–liquid interface and liquid film thickness were tracked using the VOF model.
Systematic parameter analysis: Spray height, tube diameter, Reynolds number, inlet temperature, and R32 mass fraction were compared.
Minimum film-thickness location: The high-dryout-risk region was identified near Φ = 130°–140°.

Proposed Method and Working Mechanism

1. Two-dimensional numerical model

A 2D model was established for sheet-flow evaporation of R32/R134a film sprayed onto a horizontal tube. Liquid inlet, gas inlet, pressure outlet, symmetry boundaries, and no-slip tube wall were defined.

2. Thermophysical properties and mixing rules

Properties of R32 and R134a were obtained from REFPROP and fitted as functions of temperature. Mixture density, viscosity, thermal conductivity, specific heat, and surface tension were calculated using appropriate liquid- and gas-phase mixing rules.

3. Governing equations and phase change

The VOF model was used to track the gas–liquid interface. The SST k-ω turbulence model, CSF surface tension model, multicomponent mass transfer model, and latent heat model were coupled to simulate flow, heat transfer, and mass transfer.

4. Film thickness evaluation

Local liquid film thickness was calculated from the distance between the gas–liquid interface defined by VOF = 0.5 and the tube center. Average film thickness was obtained by integrating local film thickness along the circumferential direction.

Main Findings

Velocity-field characteristicsTangential velocity was approximately one order of magnitude larger than normal velocity, so film flow was mainly governed by tangential velocity. Tangential velocity increased with spray height.

Wall shear stressWith increasing circumferential angle Φ, wall shear stress first increased and then gradually decreased. The maximum wall shear stress appeared near Φ = 130°–140°.

Average film-thickness trendsIncreasing inlet temperature, spray height, and tube diameter reduced average film thickness. Increasing Reynolds number and R32 mass fraction in the liquid phase increased average film thickness.

Effect of spray heightAt H = 4 mm, local film thickness first decreased and then increased with Φ. At H = 6–13 mm, it first increased slightly, then decreased, and finally increased sharply near the wake region.

Effect of tube diameterA larger tube diameter produced a thicker film near the impact region but a thinner film in the downstream region. Increasing tube diameter moved the minimum film-thickness location forward.

Effect of Reynolds numberIncreasing Re made the circumferential film distribution more uniform. The Φ corresponding to the minimum film thickness did not change significantly.

Minimum film-thickness locationThe minimum local film thickness mainly occurred near Φ = 130°–140°, indicating that this region is critical for dryout prevention.

Future Prospects

This study clarified the governing factors of film thickness distribution and the location of minimum film thickness during falling-film evaporation of R32/R134a outside a horizontal tube.

Future work should include direct experimental validation, three-dimensional tube-bundle models, liquid redistribution between adjacent tubes, practical refrigeration-cycle conditions, heat transfer models including dryout occurrence, and extension to other low-GWP refrigerant mixtures.

Potential Applications

The findings are useful for designing high-efficiency and low-charge evaporators.

Refrigeration systemsHeat pump evaporatorsHorizontal-tube falling-film evaporatorsLow-GWP refrigerant mixturesLow-charge heat exchangersDryout prevention design

Summary

This study numerically analyzed falling-film evaporation of R32/R134a non-azeotropic refrigerant outside a horizontal tube and evaluated film flow and liquid film thickness distribution.

Film thickness is strongly affected by spray height, tube diameter, Reynolds number, inlet temperature, and mixture composition. The minimum film thickness appears near Φ = 130°–140°, making liquid-film maintenance in this region essential for evaporator performance and dryout prevention.

          Conclusion：
          For R32/R134a falling-film evaporation outside a horizontal tube, accurate prediction of liquid film thickness is essential for high-efficiency, low-charge evaporator design, especially considering the minimum film-thickness region near Φ = 130°–140°.
        