Ammonia is an environmentally safe and low-cost natural refrigerant which has been in use for more than a century. The Kigali amendment phase-out of HFCs has further strengthened the case for ammonia. The limitations of ammonia are its toxicity and flammability at higher concentration. However, these drawbacks can be overcome by designing systems with low charge.
In pursuit of this effort, a direct expansion of ammonia study was conducted on 3-D structured enhanced surface tube bundle with saturation temperature range -2 to -20 ºC. The results showed that heat transfer coefficient increased with heat flux and saturation temperature. The effect of exit degree of super heat on heat transfer coefficient was observed at all saturation temperatures and heat fluxes. The trend was typical as observed in a classic in-tube two phase convective boiling. Heat transfer coefficient dropped with exit superheat (quality) because of the diminishing effect of boiling and dominance of single-phase sensible regime. A widespread disagreement amongst the well-known correlations with the current results was observed as shown in the graphs below.
Comparison of present study with previous flooded studies at saturation of -2°C
Comparison of present study with previous flooded studies at saturation of -20°C