Abstract
An experimental facility was designed to measure pressure drop and heat transfer coefficient during flow boiling of azeotropic refrigerants in horizontal tubes. The apparatus is made of the refrigerant circuit, including the test section; the water circuit, to provide the power for evaporation; the glycol-water circuit, to fix the refrigerant pressure. The test section is a counterflow tube-in-tube heat exchanger (refrigerant inside, water outside). Plant assessment involved the measurement of pressure drop and heat transfer during R134a flow boiling in a smooth tube (outer diameter 9.56mm, inner diameter 8.92mm). Such a case study is a benchmark for the availability of a wide experimental database in the literature, which has been also summarized in many correlations. Then further tests involved a microfin tube. The experimental conditions were: evaporation temperature, 5°C; mass flux, 111÷333kg/m2s; average quality, 0.15÷0.93; heat flux, 8.8kW/m2 and 17.6kW/m2. The uncertainty affecting the pressure drop and the heat transfer coefficient resulted lower than 1% and 5% respectively. The comparison with the literature shows satisfactory agreement with the major findings and enables using the data for the assessment of the existing models to predict both the pressure drop and the heat transfer coefficient.
Highlights
HVAC technologies are continually evolving to meet more and more severe requirements in terms of energy efficiency and environmental impact
An experimental facility was designed to measure the pressure drop and the heat transfer coefficient during flow boiling of azeotropic refrigerants in horizontal tubes.Plant assessment involved the measurement of pressure drop and heat transfer during flow boiling of R134a in a smooth tube
Afterwards a K-type thermocouple checks the water temperature calming section test section pressure drop length heat transfer length refrigerant inlet pressure tap water outlet water - refrigerant connection water inlet refrigerant outlet pressure tap and the flowenters in a plate heat exchanger, an P.I.D. driven electric heater sets the inlet temperature in the test section such that the power transfer causes a ±2°C temperature change of the water flow
Summary
HVAC technologies are continually evolving to meet more and more severe requirements in terms of energy efficiency and environmental impact. The heat transfer takes place in a tube-in-tube heat exchanger (water in the annulus, refrigerant in the inner duct), named test section (Figure 2), thermally insulated from the surroundings with 100 mm thick shell of rubber foam. The flowenters in a plate heat exchanger (if cooling is required glycol is flushed in the other side), an P.I.D. driven electric heater (it is made of two elements: 1kW and 5kW, it is possible to use both or only one) sets the inlet temperature in the test section (checked by a K-type) such that the power transfer causes a ±2°C temperature change of the water flow (inlet and outlet temperature are provided by two groups of 3 K-type thermocouples connected in series, uncertainty 0.1K). The reference junction of each thermocouple (K-type, uncertainty 0.1K) is inserted in a Dewar flask filled with melting ice
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