In this paper, two sets of experimental devices have been established for studying the falling film evaporation outside a horizontal tube and condensation inside a horizontal tube, with three different tube arrangements and seawater as the experimental fluid. The rules of the overall heat transfer coefficient (K) are determined by changing saturation temperature (T), Reynolds number (Re), total temperature difference (ΔT), inlet steam velocity (v_s), and vapor quality (x). Experimental results indicate that the overall heat transfer coefficient of the rotated square arrangement (K_rs) is higher than that of the triangular arrangement (K_t), and the square one (K_s) is the lowest with less impact on Re, T, ΔT, v_s, or x changing. With Re increasing, the K increases first and then declines, especially when Re > Re_cr, K_rs and K_t are almost the same. The increment of T and ΔT leads to the decreasing of the K, while v_s has little effect on the K. The growth of x contributes a lot to the increment of the K. In the evaporator, the vapor quality contributes a lot in the horizontal direction, while Re is more influential in the vertical direction.

Rail transit as a high-energy consumption field urgently requires the adoption of clean energy innovations to reduce energy consumption and accelerate the transition to new energy applications. As an energy-saving fluid machinery, the ejector exhibits significant application potential and academic value within this field. This paper reviewed the recent advances, technical challenges, research hotspots, and future development directions of ejector applications in rail transit, aiming to address gaps in existing reviews. (1) In waste heat recovery, exhaust heat is utilized for propulsion in vehicle ejector refrigeration air conditioning systems, resulting in energy consumption being reduced by 12~17%. (2) In vehicle pneumatic pressure reduction systems, the throttle valve is replaced with an ejector, leading to an output power increase of more than 13% and providing support for zero-emission new energy vehicle applications. (3) In hydrogen supply systems, hydrogen recirculation efficiency exceeding 68.5% is achieved in fuel cells using multi-nozzle ejector technology. (4) Ejector-based active flow control enables precise ± 20 N dynamic pantograph lift adjustment at 300 km/h. However, current research still faces challenges including the tendency toward subcritical mode in fixed geometry ejectors under variable operating conditions, scarcity of application data for global warming potential refrigerants, insufficient stability of hydrogen recycling under wide power output ranges, and thermodynamic irreversibility causing turbulence loss. To address these issues, future efforts should focus on developing dynamic intelligent control technology based on machine learning, designing adjustable nozzles and other structural innovations, optimizing multi-system efficiency through hybrid architectures, and investigating global warming potential refrigerants. These strategies will facilitate the evolution of ejector technology toward greater intelligence and efficiency, thereby supporting the green transformation and energy conservation objectives of rail transit.

In the new era of ejector technology entering into system scale from device scale. The methodology of regulation mode of an adjustable ejector has been summarized in this paper. The similarity in characteristics of qualitative and quantitative regulation under different operating modes are researched. The results show that both qualitative and quantitative regulation have the characteristics of ‘constant capacity’. Three different ‘constant-capacity’ phenomena in ejectors have been found, and the similarity indicates that the turbulent flow in ejectors has some statistical invariances in the form of mass flow rate of jetting flow. The ejector has the similar entrainment ability by increasing primary pressure by 43% in the qualitative regulation mode or enlarging the nozzle throat opening by 47% in the quantitative regulation mode under critical mode with similar underexpanded state. The advantages of adjustable ejectors have been promoted, in particular ±5% regulation accuracy or constant regulation ability, discharging improved ability by 80% in flow rate or 10% in pressure lifting ability in subcritical mode or primary flow rate saving ability by 25% or 13% in pressure lifting ability in critical mode.

As an energy-saving fluid machinery component, the ejector holds significant potential for promoting energy conservation and sustainable transformation in aerospace. This review synthesizes recent progress, identifies persistent challenges, and outlines future directions for ejector technology in this field, addressing a gap in existing reviews. (1) In aero-engine systems, performance faces constraints from high-speed compression effects and flow losses. These systems require optimized design across a wide range of speeds. A mixed configuration incorporating a blade mixer achieved a 5~7% thrust increase under static conditions. (2) In high-altitude test facilities, transient start-up and flow instability under off-design conditions demand more precise models and control strategies. An alternative solution using a second throat exhaust diffuser reduced the start-up time by 50~70%. (3) In rocket-based combined cycle engines, development is limited by thermal choking, mode transition, and combustion-flow coupling issues. Optimization of the rocket layout and geometric throat increased the bypass ratio in ejector mode by 35% and improved the specific impulse by 12.5%. Future efforts should focus on constructing multi-physics coupling numerical simulation models for ejectors, analyzing unsteady flow behavior and thermal effects within ejectors, and developing performance optimization strategies based on intelligent control. These approaches are expected to enhance the engineering applicability and system efficiency of ejector technology in the aerospace field, which is increasingly focused on energy conservation and sustainable transformation.

Based on the pinch theory and energy cascade utilization principle, the performance of a multi effect shipboard vertical tube climbing film desalination system combined thermal vapour compression with different preheating configuration has been analyzed. The mathematical model is established for thermodynamic simulation, in which various thermodynamic losses caused by boiling point elevation and pressure drop are considered, and the effective heat transfer temperature difference and temperature difference of hot side are used to analyze the influences of heating steam temperature, final effect evaporation temperature, and concentration ratio on thermal performance including gained output ratio (GOR), specific heat transfer area for different preheating configuration. The results show that the internal average effective heat transfer temperature difference determines characteristics of multi-effect distillation system. While for system with similar average effective heat transfer temperature, the temperature difference of hot side is the controlling parameter. And the energy cascade utilization principle shows the controlling attribute in multi-effect distillation thermodynamic system, as GOR is improved mainly due to utilization of the secondary energy and residual energy of the heating steam for preheating process.