The evaluation of time variation global warming effects, TWPA and CWP, for CFC alternatives

Evaluations methods of global warming are presented by considering the direct warming effect of chemical compounds and of decomposed compounds, warming effect due to the formation of troposphere ozone, and the cooling effect due to the decomposition of stratosphere ozone. It is easy to take account the stabilization of global warming gases concentration in the atmosphere, as those methods can conduct the time variations analysis. The methods are named Total Warming Prediction Analysis (TWPA) and Composite Warming Potential (CWP). The evaluation of Mobile Air Conditioning refrigerant is presented as an example of application of our method.     

The effect of vapour super-heating on hydrocarbon refrigerant condensation inside a brazed plate heat exchanger

This paper investigates the effect of vapour super-heating on hydrocarbon refrigerant 600a (Isobutane), 290 (Propane) and 1270 (Propylene) condensation inside a brazed plate heat exchanger.Vapour super-heating increases heat transfer coefficient with respect to saturated vapour, whereas no effect was observed on pressure drop.The super-heated vapour condensation data shows the same trend vs. refrigerant mass flux as the saturated vapour condensation data, but with higher absolute values. A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 15-18 kg m⁻² s⁻¹ depending on refrigerant type. The super-heated vapour heat transfer coefficients are from 5% to 10% higher than those of saturated vapour under the same refrigerant mass flux.The experimental heat transfer coefficients have been compared against Webb (1998) model for forced convection condensation of super-heated vapour: the mean absolute percentage deviation between the experimental and calculated data is ±18.3%.HC-1270 shows super-heated vapour heat transfer coefficient 5% higher than HC-600a and 10-15% higher than HC-290 together with total pressure drops 20-25% lower than HC-290 and 50-66% lower than HC-600a under the same mass flux.     

Choked flow mechanism of HFC-134a flowing through short-tube orifices

This paper is a continuation of the author’s previous work. New experimental data on the occurrence of choked flow phenomenon and mass flow rate of HFC-134a inside short-tube orifices under choked flow condition are presented. Short-tube orifices diameters ranging from 0.406 mm to 0.686 mm with lengths ranging from 1 mm to 3 mm which can be applied to a miniature vapour-compression refrigeration system are examined. The experimental results indicated that the occurrence of choked flow phenomena inside short-tube orifices is different from that obtained from short-tube orifice diameters of greater than 1 mm, which are typically used in air-conditioner. The beginning of choked flow is dependent on the downstream pressure, degree of subcooling, and length-to-diameter ratio. Under choked flow condition, the mass flow rate is greatly varied with the short-tube orifice dimension, but it is slightly affected by the operating conditions. A correlation of mass flow rate through short-tube orifices is proposed in terms of the dimensionless parameters. The predicted results show good agreement with experimental data with a mean deviation of 4.69%.     

Flow regimes and two-phase pressure gradient in horizontal straight tubes: Experimental results for HFO-1234yf, R-134a and R-410A

Two-phase flow regime visualizations of HFO-1234yf and R-134a in a 6.70 mm inner diameter glass straight tube have been simultaneous investigated by top and side views with a high speed high resolution camera. No major difference was observed between both refrigerants. HFO-1234yf flow regimes were satisfactorily predicted by the Wojtan et al. [1] flow pattern map. In addition, 819 pressure drop data points measured during two-phase flow of refrigerants HFO-1234yf, R-134a and R-410A in horizontal straight tubes are presented. The tube diameter (D) varies from 7.90 to 10.85 mm. The mass velocity ranges from 187 to 1702 kg m⁻² s⁻¹ and the saturation temperatures from 4.8 °C to 20.7 °C. The results are compared against 10 well-known two-phase frictional pressure drop prediction methods. For the entire database, the best accuracy is given by the method of Müller-Steinhagen and Heck [2] with around 90% of the data predicted within a ±30% error band. An analysis was carried out on the maximum pressure gradient and on the corresponding vapor quality. A statistical analysis for each flow regime was also carried out.