Heat Transfer and Pressure Drop of R1123/R32 Flow in Horizontal Microfin Tubes During Condensation and Evaporation

R1123 (tri fluoro ethylene; CF2 = CHF) has a GWP of approximately 1, a normal boiling point of -59 °C and a critical temperature of 59 °C, those are considered to be desirable for air condition systems. However, the concentration is limited below 60% because of the instability. Therefore, using this substance as a mixture coupled with R32 is attempted. In this study, heat transfer characteristics in a horizontal microfin tube are assessed for the new candidate of low GWP refrigerant mixture R1123 / R32 (40 / 60 mass%). Of the tested microfin tube, an outer diameter, an equivalent inner diameter, a number of fins, and a helix angle are respectively 6 mm, 5.21 mm, 60, and 18 deg. The heat transfer coefficient and pressure drop during condensation and evaporation process are experimentally quantified at saturation temperatures of 40 °C and 10 °C, respectively, and compared to those of R32 alone. The condensation heat transfer coefficient of R1123 / R32 (40 / 60 mass%)　was somewhat lower than that of R32; while, the evaporation heat transfer coefficient was almost comparable to R32. The temperature glide of R1123 / R32　(40 / 60 mass%) is approximately 1 K. Consequently, the mass transfer resistance caused by volatility difference was not obvious in heat transfer coefficient. On the other hand, the pressure drop during both process of condensation and evaporation was obviously lower than that of R32 because vapor density of R1123 / R32 (40 / 60 mass%) is higher than that of R32 alone. From the aspect of heat exchanger designing, the tested mixture offers good solution to shifting lower GWP

Thermodynamic Assessment of High-Temperature Heat Pumps for Heat Recovery

To reduce energy consumption utilizing heat recovery systems is increasingly important in industry. The screening of refrigerants and primary energy efficiency are mainly discussed with a case study for high-temperature heat pump cycles in this study. The overall coefficients of performance of four different cycle configurations to increase temperature of compressed water up to 160 oC using waste heat of 80 oC are calculated and thermodynamically compared for selected refrigerants, R717, R365mfc, R1234ze(E), and R1234ze(Z). The calculation results suggest that a multiple stage cycle drastically reduces throttling loss in expansion valve and exergy loss in condensers, and consequently achieves the highest overall coefficient of performance (COP) among the calculated cases with a refrigerant R1234ze(Z) having the critical temperature about the target outlet water temperature. A cascade cycle using R1234ze(Z) and R365mfc performs relatively high COP and also brings many practical benefits, such as varied combination of refrigerants and lubricant oils and prevention of the liquid-back caused by cold start. At a compressor efficiency of 0.7 and an approach temperature difference in heat exchangers of 2 K, the calculated overall COP is ranging from 4.3 to 4.94. This is corresponding to the primary energy efficiency 1.62 to 1.83, when the transmission-end-efficiency of electric power generation is 0.37. Even where the compressor efficiency of 0.61 and the approach temperature difference of 8 K, the efficiency is above 1.3. As remarked above, the thermodynamic assessment demonstrated that the high-temperature heat pumps to recover waste heat are promising system to reduced primary energy consumption for industrial applications

The statistical physics of discovering exogenous and endogenous factors in a chain of events

Event occurrence is not only subject to the environmental changes, but is also facilitated by the events that have occurred in a system. Here, we develop a method for estimating such extrinsic and intrinsic factors from a single series of event-occurrence times. The analysis is performed using a model that combines the inhomogeneous Poisson process and the Hawkes process, which represent exogenous fluctuations and endogenous chain-reaction mechanisms, respectively. The model is fit to a given dataset by minimizing the free energy, for which statistical physics and a path-integral method are utilized. Because the process of event occurrence is stochastic, parameter estimation is inevitably accompanied by errors, and it can ultimately occur that exogenous and endogenous factors cannot be captured even with the best estimator. We obtained four regimes categorized according to whether respective factors are detected. By applying the analytical method to real time series of debate in a social-networking service, we have observed that the estimated exogenous and endogenous factors are close to the first comments and the follow-up comments, respectively. This method is general and applicable to a variety of data, and we have provided an application program, by which anyone can analyze any series of event times.Comment: 17 pages, 7 figure

Material Cost Minimization Problem for Aluminum Alloy Beam using Beam String Structure

Aluminum alloy is a light-weight material with excellent corrosion resistance but low rigidity. When the aluminum alloy is used to a girder bridge, it takes high costs owing to the increment of its stiffness. Therefore in order to reduce a material cost, the cost minimization problem was performed on beam string structure (BSS) made of the aluminum alloy material based on the results of the topology optimization. We focused on the layout of the BSS and diameter of the cable. The conducted simulation made clear the effectivity of the BSS to the aluminum alloy material for a reduction of material cost and increment of the beam span

Condensation and Evaporation of R744/R32/R1234ze(E) Flow in Horizontal Microfin Tubes

R1234ze(E) has been anticipated to become an alternative of conventional refrigerant R410A for air conditioning systems. Latest studies revealed that the COP of R1234ze(E) alone is unexpectedly lower than that of R410A, mainly caused by irreversible loss in consequent of the small volumetric capacity. To increase volumetric capacity as maintaining the global warming potential (GWP) less than 300, adding R744 and R32 into R1234ze(E) has been attempted recently. For understanding the transport phenomenon of this ternary mixture, the heat transfer coefficient and pressure gradient of in a horizontal microfin tube is experimentally investigated in this study. Experimental data of R744/R32/R1234ze(E) (4/43/53 mass%) are compared to R32/R1234ze(E) (40/60 mass%) as the combination of GWP 300; data on R744/R32/R1234ze(E) (9/29/62 mass%) are compared to R32/R1234ze(E) (30/70 mass %) as the combination of GWP 200. At average saturation temperature of 40 oC, mass flux of 200 kg m-2s-1, and heat flux of 10 kWm-2, condensation heat transfer coefficient of R744/R32/R1234ze(E) (9/29/62 mass%) is somewhat lower than that of other mixtures R744/R32/R1234ze(E) (4/43/53 mass%), R32/R1234ze(E) (40/60 mass%) and (30/70 mass%). The temperature glide of R744/R32/R1234ze(E) (9/29/62 mass%), (4/43/53 mass%), R32/R1234ze(E) (30/70 mass%), and (40/60 mass%) is approximately 18, 11, 10, and 8 K, respectively, at 40 oC. Likewise, the magnitude of decrease in heat transfer coefficient is much related to the temperature glide during condensation. Similar effects of temperature glide is seen in data of evaporation heat transfer coefficient. At average saturation temperature of 10 oC, the evaporation heat transfer coefficient of R744/R32/R1234ze(E) (9/29/62 mass%) is slightly lower than that of other mixtures. The temperature glide of R744/R32/R1234ze(E) (9/29/62 mass%), (4/43/53 mass%), R32/R1234ze(E) (30/70 mass%), and (40/60 mass%) is approximately 22, 13, 11, and 9 K, respectively. The pressure gradient of those refrigerants are almost equal, and the difference is within measurement uncertainty. The experimental pressure gradient agrees well with prediction of Kubota et al. (2001), Filho et al. (2004), Newell and Shah (2001)

Condensation and Evaporation of R134a, R1234ze(E) and R1234ze(Z) Flow in Horizontal Microfin Tubes at Higher Temperature

Hydrofluoro-olefin R1234ze(E) and the isomer R1234ze(Z) are anticipated to be environment-friendly alternatives of R134a. Especially, R1234ze(Z) is most likely to be suitable for high-temperature heat pumps in industries. The heat transfer characteristics of those refrigerants R134a, R1234ze(E) and R1234ze(Z) are experimentally compared in this study. Their heat transfer coefficient and pressure gradient in a horizontal microfin tube are measured at temperatures of 40 and 50 oC for condensation, and temperature of 30 oC for evaporation. The equivalent inner diameter, fin height, and surface enlargement to the equivalent smooth tube of the microfin tube are 5.34 mm, 0.255 mm, and 2.24, respectively. The thermodynamic and transport properties of R1234ze(Z) are calculated by Reprop (Lemmon et al., 2013) associated the fluid file provided by Akasaka (2013) that is proposed to fit the experimental data by Higashi et al. (2013) and Miyara et al. (2013). For condensation, the heat transfer coefficient and pressure gradient of R1234ze(Z) are higher than those of R1234ze(E) and R134a at mass flux of 200 kg m-2s-1 and heat flux of 10 kW m-2. This is mainly because of the higher vapor velocity due to lower vapor density, larger liquid thermal conductivity and latent heat comparing to R134a and R1234ze(E). The experimentally determined heat transfer coefficient and pressure gradient are reasonably agree with the predicted values by Cavallini et al. (2009), Yonemoto et al. (2006), and Kadzierski et al. (1998). For evaporation, at mass flux of 200 kg m-2s-1 and heat flux of 10 kW m-2, the heat transfer coefficient of R1234ze(Z) is somewhat higher than that of R134a and R1234ze(E) at only higher vapor qualities. The pressure gradient of R1234ze(Z) is notably higher than that of R134a and R1234ze(E). The experimental heat transfer coefficient and pressure gradient are agree well with predicted values by Chamra et al. (2007), Mori et al. (2002), Thome et al. (1997)

Experimental Investigation of Relative Humidity Effect on the Thermal Conductivity of Desiccant Material

Study on effective thermal conductivity (ETC) of desiccant materials is getting attention in the literature in order to optimize the operating parameters of close and open cycle adsorption cooling systems. In addition, it is an important parameter to enhance the performance of adsorption heat pump and adsorption cooling systems (AHP/ACS). Most of the desiccant materials are porous in nature, therefore, results in different ETC at different operating conditions i.e. temperature and humidity. In order to find the more precise performance expression, the combined effect of desiccant porosity, temperature and relative humidity (RH) should be considered. In this regard, many empirical and theoretical models have been presented for the estimation of ETC. Models developed in the literature are characterized by a single value at a particular temperature irrespective of humidity. Hence this study experimentally investigates the relative humidity effect on the thermal conductivity of the commercially available desiccant material i.e. AQSOA-Z05.The levels of RH were investigated in the range of 8% to 100%. The results showed that ETC of oven dry adsorbent material was 0.066 W/mK whereas it increased from 0.067-0.089 W/mK at RH of 8-100%, respectively. The ETC values increase due to the phenomena of pores filling by water vapor adsorption. It also showed that pore filling incorporate the change in the mean free path and it varied from 6.93-0.55μm at RH range 8-100%, respectively. Consequently, an empirical correlation has been presented which can predict the effective thermal conductivity at different levels of RH

Enhancement of R1234ze(Z) Pool Boiling Heat Transfer on Horizontal Titanium Tubes for High Temperature Heat Pumps

Low GWP refrigerant R1234ze(Z) is promising candidate of refrigerant used in industrial high temperature heat pumps. The heat exchangers of such heat pumps designed for waste heat recovery systems is exposed to the exhaust containing acid substances. Using titanium as the material can be one solution to prevent oxidation. In this study, pool boiling heat transfer characteristics outside of horizontal titanium tubes were experimentally investigated for R1234ze(Z) at relatively higher temperatures. A plain tube and three enhanced tubes having different fin geometry were tested in a pressure vessel and the bubble behavior was observed. The experiment covers saturation temperatures from 10 to 60 °C and heat fluxes from 0.55 to 79.8 kWm-2. With the plain tube, it was confirmed that the measured heat transfer coefficients on the plain tube agree with the calculated heat transfer coefficients by Jung et al. correlation proposed for other HFC refrigerants within ±15%. The notable difference to the typical material copper is thermal resistance over the tube wall. Because the thermal conductivity of titanium is only 19 W m-1K-1 (that of copper is approximately 360 W m-1K-1), the thermal resistance is onsiderable. Comparing to the plain tube, the tested enhanced tube exhibited 2.8 to 5.1 times higher heat transfer coefficient, on average, in the test range. This could compensate the disadvantage in thermal conductivity for titanium. The enhancement ratio predominantly depends on the saturation temperature and wall heat flux. At conditions of the higher saturation temperatures and lower heat fluxes, where smaller bubbles were observed, the test tube having smaller fin spaces exhibits higher heat transfer coefficient. The experimental results remark the importance of fin geometry optimization to the operation conditions

Comparison on Evaporation Heat Transfer between R32/R1234yf and R32/R1234ze(E) Flowing in Horizontal Microfin Tubes

Refrigerant mixtures R32/R1234yf and R32/R1234ze(E) are considered to be the low GWP alternatives of R32 and R410A for air conditioners. However, according to the recent reports, the severe heat transfer degradation is encountered during the evaporation process. This implies that much larger heat exchangers are required to maintain the COP and cooling/heating capacity with R32/R1234yf and R32/R1234ze(E). Therefore, the effects of mixture component and composition on the heat transfer degradation is experimentally investigated in this paper. Heat transfer coefficient of those two mixtures and their components (i.e., R32, R1234yf and R1234ze(E) alone) are experimentally quantified with horizontally set copper microfin tubes of 6.00 mm in outer diameter having 48, 58, and 64 fins of 0.26 mm in height and 19 ° in helical angle. The evaporation test was conducted at an average saturation temperature of 10 °C, a heat flux of 10 kW m-2, and mass fluxes from 150 to 400 kg m-2s-1. The heat transfer is degraded most at the composition where the temperature glide and concentration difference between vapor and liquid phases are maximized. This suggest that the relevance of the heat transfer degradation and mass transfer resistance caused by the concentration boundary layer and the additional sensible heat transfer. Although the heat transfer coefficients of R1234yf and R1234ze(E) alone are comparable, the magnitude of heat transfer degradation is obviously severer for R32/R1234ze(E) than that for R R32/R1234yf. This can be explained with the larger concentration difference and the temperature glide of R32/R1234ze(E) that those of R32/R1234yf. With increasing mass flux and number of fins, the heat transfer degradation is mitigated somewhat. It appears to be that mixing of concentration boundary layer contributes to recover the evaporation heat transfer. For the real possibility to use R32/R1234yf and R32/R1234ze(E) in air conditioners, the heat transfer recovery or enhancement could be the key technology

Theoretical and Experimental Analysis of Desiccant Air Conditioning System for Storage of Agricultural Products

The study emphasizes on the use of desiccant air conditioning (DAC) system for the storage of agricultural products. The chilling sensitivity of the tropical fruits and vegetables makes this system more promising for their optimal storage. The desiccant air conditioning system assisted by Maisotsenko cycle evaporative cooler is proposed in the study to achieve the latent and sensible load of air conditioning. In this regard, the dehumidification evaluation of the honeycomb like polymer based hydrophilic desiccant blocks are carried out by the means of an open-cycle experimental unit. The representative ideal storage zones of three temperature and relative humidity compatible groups of fruits and vegetables are established on the psychrometric chart on the basis of published data. The ideal DAC cycle analysis is accomplished at low regeneration temperature (55°C) for case-I (T = 31°C; RH = 21%) and case-II (T = 13°C; RH = 70%). The dehumidification analysis of the desiccant blocks recommended the time ratio between regeneration and dehumidification modes as 1:1 and 2:3 for the case-I and case-II respectively. The suggested time ratios ensure the dehumidification of the process air up to 2 g/kg of dry air and 4 g/kg of dry air in case-I and case-II respectively. The COP of the system was calculated as 0.90-0.43 and 0.55-0.25 at 30-90 minutes dehumidification with regeneration heat supplies of 1.7-2.3 kW and 2.5-3.5 kW in case-I and case-II respectively. The promising dehumidification by the desiccant blocks resulted in achieving the latent load itself followed by flat plate heat exchanger and Maisotsenko cycle evaporative cooler to achieve the sensible load. However, in case of high sensible loads hybrid DAC system is being recommended in this study