This study addresses the limited condensation data for low-GWP R1234yf in staggered-fin plate heat exchangers (SFPHEs) for new-energy-vehicle thermal management. Condensation-side heat transfer coefficients and frictional pressure drops were measured using a modified Wilson plot method over mass flux G = 100–300 kg m⁻² s⁻¹, heat flux q = 12–18 kW m⁻², saturation temperature Tsat = 55–65°C (Psat ≈ 1.53–1.91 MPa), and mean vapor quality x̄ = (xin + xout)/2 covering 0–0.9. These ranges correspond to high-ambient NEV heat-pump and chiller condenser duty. Vapor-quality-dependent regime transitions govern performance: high x̄ sustains relatively high HTCs, intermittent flow causes the steepest decline, and at low x̄ the flow is liquid-dominated and intermittent but remains shear-dominated. Mass flux amplifies frictional losses; heat flux has a secondary effect; higher saturation temperature degrades HTC but lowers pressure drop. The fitted Nu and ff correlations describe 88.6% and 85.6% of the same data within ±20%. These percentages measure goodness of fit, not independent validation. The results provide guidance for sizing compact R1234yf SFPHE condensers.
Marine exhaust gas is a promising heat source for absorption refrigeration, but its temperature and heat capacity vary with engine load and should therefore be treated as a finite heat source. This study investigates a marine exhaust-driven single-effect LiBr–H2O absorption chiller using a framework that distinguishes the available exhaust heat, maximum recoverable heat, actual generator heat input, and useful cooling capacity. The effects of available exhaust heat and generator conductance were mapped to identify source-limited and heat-transfer-limited operating regions. Increasing exhaust heat availability did not proportionally improve cooling performance when generator conductance was insufficient. These limitations propagated through refrigerant generation, solution concentration difference, circulation ratio, and cooling capacity. An engineering desorption-efficiency indicator was defined to quantify the effective utilization of generator heat for refrigerant generation. A heat sufficiency number (HSN), expressed as the ratio of maximum recoverable heat to the generator heat required for a 15 kW target cooling capacity, was also introduced to evaluate source-to-demand thermal matching. Operating maps based on generator set temperature and seawater inlet temperature were used to evaluate cooling performance and operating feasibility. The results showed that high cooling output does not necessarily coincide with adequate heat sufficiency and crystallization safety. Practical operation should therefore be selected within the admissible operating domain while satisfying both heat-sufficiency and crystallization-margin requirements. The proposed framework provides a systematic method for generator sizing, operating-condition selection, and feasibility assessment of marine waste-heat-driven absorption chillers.
TEARSs represent the most advanced and efficient generation for utilizing solar thermal energy or waste heat. The heat and mass transfer processes within system components are highly complex, which leads to a significant exergy destruction. Therefore, first law of thermodynamics is inadequate for a comprehensive performance assessment and design optimization. This study presents an integrated thermodynamic framework combining thermal modeling, exergy analysis and multi-objective optimization of a series flow triple-effect LiBr/H2O absorption refrigeration system under steady state conditions. The study included a parametric analysis to assess the impact of operating temperatures on energy and exergy performance within viable operating ranges. To determine the optimal operating conditions, a multi-objective optimization was performed using NSGA II algorithm to simultaneously maximize COP and minimize total exergy destruction. Pareto front was used to illustrate the competing between the two objectives, while Standard Achievement Function (ASF) was employed as a tool to select the preferred operating point. ASF, with weights of (0.3) for exergy destruction and (0.7) for COP, was used to select preferred solution from Pareto front, where ASF value of the selected solution was (0.154). The results showed that the optimal solution was achieved at THTG=179.16∘C, TMTG=130.01∘C, TLTG=83.28∘C, TC=37.72∘C, TA=33.01∘C, and TE=9.99∘C. The model achieved a COP of (1.87), Exergetic efficiency of (15.11%), and total exergy destruction of (794.65 kW), at refrigeration effect (3516.85 kW). Exergy destruction Analysis indicated that the highest rates were concentrated in HTG (20.31%), MTG (18.82%), and LTG (17.43%), respectively. Absorber and evaporator contributed (14.41%) and (11.24%), respectively.
High temperature heat pumps are a promising option for decarbonising industrial process heat, but conventional reversed Rankine systems become difficult to extend to very high delivery temperatures because of working fluid critical temperature limits, high pressure ratios, excessive compressor discharge temperatures, and fluid stability constraints. This study evaluates a cascaded heat pump cycle in which a water-based reversed Rankine lower stage is coupled with a reversed Brayton top stage to extend the useful heat delivery envelope of a reversed Rankine cycle. The cascaded reversed Rankine–reversed Brayton cycle is compared with a simple reversed Brayton cycle under identical external boundary conditions. Both systems are designed to deliver 1 MW of heat to a sink with inlet temperatures between 250°C and 400°C and glide temperatures between 50°C and 200°C. For each operating condition, the intermediate coupling temperature, defined by the reversed Rankine condensation temperature, is optimised to maximise the overall COP. The results show that the optimum intermediate coupling temperature increases with sink inlet temperature and decreases with glide temperature. The cascaded cycle improves COP mainly at high sink inlet temperatures and low-to-moderate glide, reaching about 15% improvement at 400°C and 50°C glide under the baseline design. However, this beneficial region also coincides with increasingly severe water-vapour compression conditions in the reversed Rankine stage. The results show that a reversed Brayton top stage can extend the heat delivery range of a reversed Rankine cycle, but the advantage over a simple reversed Brayton cycle is conditional.
The current study examines the impact of variations in hot exit area on flow dynamics. An approach that adjusted the plug angle to reduce hot exit area was adopted to induce flow separation. Consequently, the effects of hot exit area on mass-fraction and flow dynamics were analyzed by evaluating swirl intensity, pressure and temperature gradients, helicity, exergy, and shock formation. The variation in mass-fraction drives a supersonic flow in a cold nozzle due to a higher pressure gradient. The computational model, based on Reynolds-Averaged Navier-Stokes equations solved using Re-Normalization-Group k-epsilon turbulence model, was employed. The limitations of swirl intensity to analyze the radial flow were identified, and introduced helicity for correlate and assess energy separation, radial flow, and bifurcation point. helicity shows a value of zero at the bifurcation points (axial positions of 5.8Dto and 8.2Dto) for mass-fractions of 0.06 and 0.16, respectively. At a mass-fraction of 0.06, reduction of the helicity to a value of 1.5 × 107 m/s2 is suspected due to the radial flow mixing with the return flow, and further, the return flow forms a whirlpool effect at the cold exit. At an optimal mass-fraction of 0.42, the cold exit temperature reached 122K, exhibiting a supersonic flow profile. Further optimization of the inlet and other design parameters is necessary to maximize cooling at the cold exit under supersonic flow conditions. Additionally, the computational results show a shock train phenomenon at the cold nozzle, influenced by the cold mass-fraction, warranting further experimental validation.
