اصول اولیه مدل شبیه سازی عملکرد حرارتی هیدرولیک فن در عرضه مبدل های حرارتی لوله باله

This paper outlines a novel first-principles mathematical model to simulate the thermo-hydraulic behavior of compact fan-supplied tube-fin heat exchangers for light commercial refrigeration applications, i.e., with heat duties ranging from 0.5 to 2.0 kW. The model is based on the mass, momentum and energy conservation equations applied to both the refrigerant and air streams. The model predictions were compared with experimental data taken at several operating and geometric conditions. It was found that the model predictions for the air-side heat transfer and pressure drop were very close to the experimental data with maximum deviations of ±10% and ±15%, respectively. The model was employed to assess the thermal-hydraulic performance of a gas cooler running with supercritical CO2 as working fluid.

The annual Brazilian energy consumption due to air conditioning and refrigeration equipment is approximately 50,000 GWh, which corresponds to almost half of the Itaipu hydropower plant capacity [1]. Such a figure not only highlights the needs for better energy utilization practices, but also points out the urgency for more efficient refrigeration systems. It is well-known that the energy consumption of refrigeration systems is mostly due to the irreversible thermodynamic losses that take place in each of the system components, among which the heat exchangers present the higher performance/cost ratio. Most of the heat exchangers used in small-size refrigeration and air conditioning systems are of the tube-fin type, where the air flows externally over extended finned surfaces whereas the refrigerant flows internally along the tubes. Performance analysis of this type of heat exchanger is usually assessed experimentally, thus requiring specific test rigs, prototype samples and trained technical personnel. A faster and less costly alternative is to employ mathematical models to simulate the thermo-hydraulic behavior of the heat exchanger coils. The simulation not only rationalizes the number of prototypes and experiments needed, but also permits the heat exchanger optimization based on both component-level (e.g., j and f) and system-level (e.g., COP) performance indicators.

A numerical simulation model for fan supplied compact tube-fin heat exchangers was presented. The model predictions for the temperature profiles, heat transfer rate and the air-side pressure drops were compared with experimental data obtained in-house using a purpose-built testing facility. It was observed that the model is able to predict the overall performance of heat exchangers in terms of heat transfer rate and pressure drop with errors within 10% and 15% bands, respectively. The model predictions for the temperature profiles followed closely the experimental trends. Additional comparisons were carried out using experimental data obtained in the literature for a gas cooler that operates with supercritical CO2 as working fluid. It was observed that the model provides satisfactory results for both the heat transfer rate and pressure drop. Finally, the model was used to analyze the thermo-hydraulic behavior of a fan-supplied gas cooler, showing that reductions of 20% in the heat transfer surface decreased the heat transfer rate by 1% only. Although originally developed for condensers and gas coolers, the model can be easily extended for evaporators if a proper heat transfer correlation for the evaporating flow is provided.