Experimental Investigation of Liquid and Vapor Mass Flows in a Parallel Tube Evaporator

One of the major challenges in the evaluation of an evaporator heat exchanger in refrigeration cycles is to provide reasonable measurement data. Especially if the evaporator is multi-flow and therefore the measurement of the mass flow at one location and the evaluation of the overall energy balance is not sufficient for the characterization of the heat exchanger. In the present work, an evaporator of a capacity of approx. 1.2 kW, consisting of a distributor, a collector and four parallel evaporator tubes in which heat was introduced, was experimentally investigated. The four evaporator tubes made of stainless steel with an inner diameter of 2 mm, an outer diameter of 4 mm and a length of 800 mm were led out of the distributor and into the collector at a right angle at a distance of 90 mm from each other. All tubes form a plane. To keep the geometry of the joints as uniform as possible, commercially available fittings were used.
The evaporator was investigated as part of a refrigeration cycle with several adjustment possibilities like mass flow, subcooling, and pressure. To determine the mass flow and vapor quality distribution within the heat exchanger, each evaporator tube was divided into two heating sections of 400 mm and 200 mm length respectively with an intermediate measuring section of 200 mm for pressure and temperature. The first sections were heated by variable electrical heat loads, uniformly distributed over a length of 345 mm. The heat load was adjusted so that a slight superheating of approx. 4 K occurred at the outlet. In the second sections a constant electrical heat load was uniformly applied over a length of 150 mm. Assuming that a thermal equilibrium between the phases in the first sections is established, the heat loads can be used to draw conclusions about the respective mass flows of liquid refrigerant. In the second sections, the constant heat flow can be used to draw conclusions about the different total mass flows due to the different outlet temperatures. The measurements were carried out in our own laboratory for different vapor qualities from x=0 to x=0.3 and different overall total mass flows of the cycle ranging from 5, 7.5 and 10 kg/h to partly 20 kg/h. During the measurements, different inclinations of the evaporator relative to the horizontal plane were realized as well, so that a large number of measurement results were finally recorded. The measurement setup allows the evaluation of liquid and vapor refrigerant mass flow rates in each evaporator channel and makes a comparison between the sum of the four individual total mass flow rates and the overall total mass flow rate of the cycle possible. The results show the influence of the variable gravity vector and flow velocity on the distribution. In the horizontal measuring point the tube furthest away from the distributor inlet exhibits the highest vapor quality of all 4 tubes, contrary to what was expected. Finally, the results are used to validate the simulation models of the industrial project partner.