Effect of ejector structural parameters on a solar-assisted ejector-enhanced heat pump dryer system

Drying is an energy-intensive process, and improving its efficiency is essential for reducing energy consumption and preserving product quality. Solar-assisted ejector-enhanced heat pump dryers (SE-HPDs) combine solar thermal input with expansion-work recovery to improve energy utilization. Unlike previous studies that mainly focused on operating parameters or fixed ejector geometries, the novelty of this work lies in experimentally resolving the coupled effects of adjustable ejector structural parameters on dynamic SE-HPD performance and establishing an area-ratio matching criterion. This study experimentally investigated a modified SE-HPD with a closed-loop drying chamber, using a hot-water loop to simulate solar thermal input. Building on a previously reported SE-HPD platform, the present work specifically addresses the unresolved geometric optimization of its adjustable ejector under dynamic drying conditions. The nozzle throat diameter, mixing chamber throat diameter, and nozzle exit position (NXP) were systematically varied to evaluate drying, heating, and ejector performance. At the optimal nozzle throat diameter of 2.21 mm, the specific moisture extraction rate (SMER), moisture extraction rate (MER), and heating coefficient of performance (COP) reached 1.76 kg·(kWh) ^-1, 1.65 kg·h⁻¹, and 3.62, respectively. Further reducing the nozzle throat diameter decreased the heating capacity by 14.8%. A mixing chamber throat diameter of 5 mm yielded an SMER of 1.63 kg·(kWh) ^-1 and an MER of 1.60 kg·h⁻¹, whereas 6 mm induced vortex formation and reduced entrainment performance by 12.3%. An NXP of 5 mm maximized the entrainment ratio at 0.52, while increasing NXP to 11 mm caused jet impingement and reduced the ratio to 0.31. The optimal nozzle throat diameter increased with the mixing chamber throat diameter, revealing a critical area ratio for geometric matching. These results provide quantitative guidance for designing and dynamically optimizing efficient heat pump drying systems.