The two-phase flow pattern of hydrocarbon working fluids on the shell side of a helically baffled heat exchanger for liquefied natural gas determines its heat transfer performance. This study tested the two-phase flow patterns of propane and ethane/propane mixtures on the shell side of a helically baffled heat exchanger using a visualization experimental method. The test results demonstrated that with the increase in vapor quality, the experimental observations sequentially included stratified flow, stratified-spray flow, and spray flow; as the mass flux of propane increased from 20 kg/(m²·s) to 40 kg/(m²·s), the transition vapor quality from stratified flow to stratified-spray flow decreased from 0.7 to 0.3, while the transition vapor quality from stratified-spray flow to spray flow decreased from approximately 1 to 0.7; when the proportion of ethane increased from 0 to 50%, the transition vapor quality from stratified flow to stratified-spray flow increased from 0.30-0.45 to 0.43-0.55, while the transition vapor quality from stratified-spray flow to spray flow increased from 0.69-0.85 to 0.83-close to 1. The existing flow pattern map for water-air mixtures was inadequate for predicting the flow patterns of hydrocarbon working fluids. A new set of flow pattern transition criteria was established with prediction deviations of approximately 6.5%, 5.5%, and 4.2% for the experimental stratified flow, stratified-spray flow, and spray flow, respectively.

Achieving low-carbon combined cooling and heating supply in distributed areas away from centralized cooling and heating networks is highly significant in the context of carbon neutrality. This study proposes a combined cooling and heating system based on an absorption heat pump, which uses a variety of clean and renewable energies, such as solar heat, geothermal, waste heat, biomass, and air-source energy, to achieve the combined cooling and heating in a wide temperature range from -20 ℃ to 90 ℃. Such systems are suitable for distributed areas, such as villages, cities, and industrial parks. The system model was constructed based on Aspen, and a prototype was developed. The prototype uses a vacuum tube collector to capture solar thermal energy and introduces natural gas as a supplementary heat source to balance fluctuations of solar energy. Multiple sets of indoor heating and cooling terminals can be driven through medium circulation and valve switching using a single set of absorption heat pumps and outdoor units. The environmental test of the prototype was performed in Jinan, and the solar thermal ratio reached 35% during the testing period. An all-weather stable energy supply was achieved by proportional control of natural gas. Moreover, a wide range of concentration adjustments was achieved by controlling the liquid level in the solution tank, enabling efficient system operation in a wider temperature range. The coefficient of performance (COP) of cooling reached 0.30-0.43 at -20 ℃ and 0.70-0.78 at 7 ℃, with cooling water temperatures varying from 30 ℃ to 20 ℃; the COP of heating reached 1.40-1.90 at 45 ℃ and 1.35-1.56 at 80 ℃, with evaporation temperature varying from -15 ℃ to 20 ℃. The study results demonstrated that introducing solar thermal energy and ambient energy recovery increased the fraction of renewable energy in the system to over 50%. Compared with the traditional method of gas furnace plus air conditioning, the annual operating cost and carbon emissions of the proposed system were reduced by over 54.3% and 44%, respectively, which has significant application potential.

Microchannel cooling, with its high heat transfer efficiency, low thermal resistance, and light weight advantages, is one of the most effective technologies for solving the problem of heat dissipation with high heat flux; however, it faces the issue of increased pressure drop. The microchannel structure determines the thermal-hydraulic performance. This study describes the research progress on single-phase liquid-cooled microchannel heat sinks in terms of domestic and international structural design to address this problem. Among them, single-phase heat dissipation structures are divided into variable cross-sectionals, flow disruption, pin-fin, double-layered, bionic, and hybrid-reinforced structures. The advantages and disadvantages of the heat transfer coefficient, pressure drop, comprehensive performance, and temperature uniformity were analyzed based on the principle of enhancing heat transfer in various structures. A cost analysis of the commonly used matrix materials and processing methods for microchannel heat sinks was conducted. Finally, we provided the prospect and development direction of microchannel heat sinks from an application perspective. The application of composite structures, integration of simulation and experimentation, advances in material science and processing technology, and the nexus of disciplines are noted as the focus of future structural research.