Evaporation test bench to investigate the heat transfer during thin-film evaporation

 

The evaporator test bench of our department is used to investigate the heat transfer during the evaporation of different refrigerants. The test bench is specialized in investigating the thin-film evaporation of water or water-alcohol mixtures on different evaporator tubes.

The test bench consists of the two vacuum-tight main components evaporator and condenser (see Figure). A heat transfer fluid flows through the evaporator tubes from the inside. The refrigerant evaporates outside the evaporator tubes by natural convection (pool boiling). Water or a water-glycol-mixture for temperatures below 0 °C is used as the heat transfer fluid. The inlet temperature and the volume flow of the heat transfer fluid are conditioned using a thermostat. The evaporator pressure - and thus the boiling temperature - can be adjusted by controlling the condenser temperature. The evaporator pressure can be kept constant during a test even when the evaporator power increases. Two operating modes are possible for the refrigerant filling level in the evaporator: (1) A quasi-stationary measurement with a continuously decreasing filling level during the measurement, and (2) a stationary measurement with discrete filling levels. The level can be determined online by evaluating camera images. The test bench is located in a climatic chamber, which can be tempered according to the boiling temperature to avoid heat exchange with the environment. Furthermore, the air in the climatic chamber can be dehumidified, thus preventing the formation of condensation on the test bench components.

The evaporator test bench has different measurement sensors to evaluate heat transfer. The evaporator power is balanced using the heat transfer fluid's temperature and volume flow sensors. The boiling temperature can either be calculated from the pressure signal or determined with various temperature sensors. The measurement data recording and test bench control are fully automated using a LabVIEW-based control program.

The most important key data of the test bench is given in the Table below. Further details of the test bench, such as dimensions or measurement sensors used, can be taken from our publication on the evaporation test bench.

Table containing key data of test bench

Evaporation mechanism	Thin-film evaporation (pool boiling)
Refrigerant	Water, alcohols, mixtures
Heat transfer fluid	Water, water-glycol-mixture
Volume flow rate heat transfer fluid	1-12 L/min
Temperature range heat transfer fluid	-10 - 40 °C
Boiling temperature	-10 - 40 °C
Boiling pressure	0,1 - 1000 mbar
Evaporation power	0,1 – 1,5 kW
Number of evaporator tubes	Until 4
Outer diameter evaporator tubes	Until 15 mm
Length evaporator tubes	510 mm

All temperature sensors were calibrated using two highly accurate Ahlborn temperature sensors. The measurement uncertainty of all temperature sensors is less than 0.05 K. The measurement uncertainty of the volume flow sensors is 0.2% of the measured value, and the measurement uncertainty of the pressure transducers is 0.1% of the measured value. All uncertainties apply to a simple standard uncertainty.

  Copyright: © Institute of Technical Thermodynamics, RWTH Aachen University