The operation of an evaporator is essentially the transfer of heat and mass of the working fluid under specific thermodynamic conditions. This involves various flow forms and their physical properties, which determine the heat exchange efficiency, operational stability, and applicability of the equipment. A deep understanding of the physical characteristics of liquid, gas, and gas-liquid two-phase flows provides a basis for evaporator selection, structural design, and operational optimization.
In the initial stage of the liquid working fluid entering the evaporator, its flow properties are mainly reflected in parameters such as density, viscosity, thermal conductivity, and specific heat capacity. Density affects the pump's delivery power and the flow velocity distribution within the pipes; viscosity relates to flow resistance and the wettability of the heat exchange surface; and thermal conductivity and specific heat capacity directly affect the sensible heat transfer rate. When the working fluid viscosity is high or contains suspended particles, it is prone to causing localized blockage of the flow channels or uneven heat exchange. Therefore, the design must consider matching the flow channel cross-section with the pumping capacity, and sometimes preheating or dilution is used to improve flowability.
As heat is added, the temperature of the liquid working fluid rises and undergoes a phase change at its boiling point, entering the gas-liquid two-phase flow stage. This is the most complex stage in terms of evaporator fluid properties. In this two-phase flow, the gas and liquid phases coexist, with a significant density difference, resulting in various flow patterns such as stratification, annular, and slug-like flow. The heat transfer and pressure drop characteristics of different flow patterns differ significantly. For example, annular flow has a large heat transfer coefficient due to the thin liquid film and high gas velocity, but if the liquid film ruptures, it can cause a sudden drop in heat transfer or even dry walls. Slug-like flow, with its alternating liquid slurries and gas pockets, easily leads to pressure and temperature fluctuations. Evaporator design must select a flow pattern conducive to stable heat transfer based on the expected operating conditions and guide the flow pattern through structures such as liquid distributors and baffles.
After evaporation, the gas phase fluid properties become dominant. Its density is much lower than that of the liquid phase, and its flow velocity increases significantly, carrying latent heat as it leaves the evaporator and enters the subsequent system. At this point, the thermal conductivity of the gas is low, and its contribution to heat transfer mainly depends on the transport of latent heat. Its specific heat capacity determines the temperature rise during subsequent condensation or compression processes. The compressibility of gases necessitates sufficient pressure margins in compressor and piping designs to prevent erosion or noise caused by excessive flow velocities.
The material properties within the evaporator are also related to the surface tension and wettability of the working fluid. Surface tension affects the spreading and thickness distribution of the liquid film on the heat exchange surface, while wettability determines whether the liquid film can uniformly cover the heat exchange surface for effective heat transfer. For working fluids prone to foaming or with abnormal surface tension, the evaporation process may generate a large number of bubbles, hindering liquid film stability, requiring defoaming or special surface treatment.
Pressure and temperature are external constraints that govern all material properties. Pressure determines the boiling point and the magnitude of latent heat of phase change, and also alters the distribution range of physical properties such as density and viscosity; temperature gradients drive sensible and latent heat transfer, and simultaneously affect the critical conditions for flow pattern transitions. Maintaining stable pressure and temperature during operation can prevent heat transfer deterioration or equipment shock caused by abrupt changes in material properties.
Optimizing the evaporator flow path and liquid distribution based on the properties of the fluid can increase the heat transfer coefficient by approximately 8% to 15% and reduce energy consumption fluctuations caused by flow pattern instability. Understanding and utilizing these properties enables more efficient and reliable heat and mass transfer under different working fluids and conditions, providing a solid physical basis for evaporator applications in refrigeration, chemical, and environmental protection fields.