Development of a refrigerant to refrigerant heat exchanger for high efficiency CO2 refrigerant cycle

The performance of CO2 cycle is worse than that of HFC refrigerants since the working pressure of CO2 cycle is higher than that of HFC refrigerants which suffers higher compression and expansion losses. To enhance the cycle performance similar extent to HFC refrigerants, refrigerant to refrigerant heat exchangers need to be applied as one of the many components equipped. Specifically, when such a heat exchanger is used as an economizer, which works between high temperature refrigerant after the gas cooler and low temperature refrigerant by reducing to the intermediate pressure, it is confirmed from the visualization that the refrigerant flow in low temperature side is observed as a vapor-liquid dispersed flow. In such a condition, the evaporating performance is low since the liquid flows in the vapor core. In this study, aiming at weight saving and compactness, a newly designed aluminum double-tube heat exchanger in which surrounding multi-ports form instead of outside annulus is used. Spiral fins are applied in the inside tube to aim for attaching liquid refrigerant flowing in vapor core to the inside tube wall. The local heat transfer coefficient is experimentally measured in a single tube and it is confirmed that this new type heat exchanger can enhance heat transfer performance

The Effect of Inner Grooved Tubes on the Heat Transfer Performace of Air-Cooled Heat Exchangers of Co2 Heat Pump System

In the CO2 heat pump system, Poly alkyl glycol (PAG) oil is commonly used for lubrication of the compressor, while it is reported that the PAG oil has influence on the heat transfer performance due to its immiscibility against CO2 within its working condition. Experimental work has been carried out to investigate the heat transfer performance of fin and tube heat exchangers using CO2 as a refrigerant with taking PAG oil into account. From previous study it was found that the heat transfer performance decreased significantly in both evaporator and gascooler conditions when oil was mixed. The present work presents experimental results on three types of air-cooled heat exchangers with smooth and inner grooved tubes. It was found that the deterioration ratio of heat transfer performance with oil was different depending on the inner surface geometry of the grooved tubes. To understand this, flow visualization inside these tubes has also been carried out through transparent section made of glass, which can withstand high pressure. It was confirmed that the oil behavior inside tubes was related to heat transfer performance. Heat transfer performance can be improved by using inner grooved tubes with the optimal pattern to remove oil away from inner surface. These tubes are effective to develop high performance heat exchangers for the CO2 heat pump system

A study of high efficiency CO2 refrigerant VRF air conditioning system adopting multi-stage compression cycle

CO2 refrigerant is expected to be use for air conditioners due to its environmental property of GWP = 1, non-flammability and low toxicity and has already been applied to hot water supply system, refrigerators and freezers. However, since cycle COP of CO2 is poorer than that of HFC in applying to air conditioner, it cannot be expected to improve the performance significantly by simply improving the efficiency of a conventional single-stage cycle and components consisting of the cycle.  Therefore, it is important to improve the cycle efficiency, especially in the performance of cooling which is a weak point of CO2. Challenges also lie in reducing weight of components such as compressor and heat exchangers resulted in weight gain to resist high pressure and to achieve high efficiency in the system. In this study, a prototype of CO2 refrigerant VRF air conditioning system is constructed and evaluates the performance. The system is adopting the four-stage compression cycle applying a new type compressor and heat exchangers and so on aimed for high efficiency cycle and reducing weight As a result of the evaluation, it is confirmed that CO2 cycle performance can be improved in similar extent to HFC refrigerant cycle. On the other hand, this high efficiency cycle consists of a lot of components and becomes more complex than HFC refrigerant cycle; therefore the size and weight of outdoor unit increases to house these components. Further technical development will be necessary to overcome these weaknesses

Characteristics of two-phase flow structures and transitions in vertical upflow

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A Stand Structure Dominated by Betula maximowicziana in the Chichibu Mountains, Central Japan

1．秩父山地の標高1,300m地点にあるウダイカンバ優占林分内に0.96haの調査区（以下，秩父区）を設置し，毎木調査から林分構造を，樹齢測定から林齢を明らかにした。2．秩父区内に出現した樹種数は39種，立木本数は1,177本/ha，BA合計は42.0m2/ha，材積は330.0m3/haであった。そのうちウダイカンバは本数が83本/ha（7.1％），BA合計が15.8m2/ha（37.5％），材積が154.7m3/ha（46.9％）であった。3．秩父区では22m以上の階層でウダイカンバが圧倒的に優占する第一層，14～22mの階層はミズメ，ミズナラ，イヌシデなどの多い第二層，および4～14mの階層はカエデ類，シデ類，アラゲアオダモ，リョウブの多い第三層の3つの階層が確認された。4．秩父区内とその周辺にある枯死木20本の樹齢測定の結果から，調査木の年輪数は83～120年の範囲にあり，そのうち近年枯死した5個体の年輪数から秩父区は約120年前に更新したものと推定された。5．秩父区と，山火事後に更新したウダイカンバ優占二次林である東京大学北海道演習林固定標準地の北演5133（3回間伐，林齢91年），北演5236（無間伐，林齢86年）を比較したところ，秩父区は本数密度と年直径成長量で北演5133に近い値を示し，平均胸高直径と材積で北演5133と北演5236より大きかった。しかし，秩父区のウダイカンバは用材として評価した場合，北演5133のそれより劣っており，秩父演習林に多い林齢100年未満のウダイカンバ二次林で用材生産をする場合には，優良木を選木してその直径成長を促すための間伐を実施する必要があると考えられた。6．秩父区ではミズナラがウダイカンバに比べて寿命が長く，耐陰性も高いことから，今後の長い遷移過程において徐々にウダイカンバが枯死し，それに代わって現在第一層，第二層にあるミズナラが優占していくものと推察された。In order to investigate the floristic composition and structure of a stand dominated by Betula maximowizciana, a research plot with an area of 0.96 ha was established at an altitude of 1,300m a.s.l. in the Chichibu Mountains, central Japan. Diameter at breast height (dbh) and the height of all the trees with dbh larger than 5.0cm were measured in the plot. Thirty-nine tree species were recorded in the plot. The number of trees, basal area and stand volume were 1,177/ha, 42.0m2/ha and 330.0m3/ha, respectively. For B. maximowicziana, these values were 83/ha (7.1%), 15.8m2/ha (37.5%) and 154.7m3/ha (46.9%), respectively. The stand was stratified into three layers, the first-layer (height of layer: hl>22m) in which the predominant species was B. maximowicziana, the second-layer (14<hl≦22m) in which B. grossa, Quercus crispula and Carpinus tschonoskii were abundant and the third-layer (4<hl≦14m) in which Acer spp. Carpinus spp., Fraxinus languinosa and Clethra barvinervis were abundant. From the age analysis of twenty dead trees sampled within the plot and its surroundings, tree ages were in the range of 83 to 120 years. From the age analysis of five sample trees, which seemed to die recently, it was estimated that this stand was regenerated about 120 years ago. Numerical values of the stand in Chichibu plot were compared to those of the stands dominated by B. maximowicziana in two plots (plot no. 5133 and 5236) in the Tokyo Univ. Forest in Hokkaido, northern Japan. They were regenerated after forest fire and the stand age of plot no. 5133 and 5236 were 91 and 86 years, respectively. Qualitative thinning has been conducted three times in the former plot but this was not the case in the latter. Tree density and mean annual diameter growth rate of the Chichibu plot were similar to those of plot no. 5133. On the other hand, mean dbh and stand volume at the Chichibu plot was larger than those of the two plots in Hokkaido. However, from the viewpoint of the commercial value of wood utilization, the morphological characteristics of the B. maximowizciana trees in the Chichibu plot are inferior to those of the trees in plot no. 5133 where qualitative thinning has been conducted. Hence, for the production of high-quality large diameter trees, qualitative thinning should also be conducted especially to B. maximowicziana stands with an age less than 100 years. These are abundant in the Tokyo Univ. Forest in Chichibu. It is thought that the B. maximowicziana trees composing the canopy layer in the Chichibu plot at present are gradually being replaced by Quercus crispula in the long course of succession, since the latter, characterized by its longer life span and higher shade tolerance, has an advantage over the former