A Critical Assessment of Two-Phase Flow Distribution in Microchannel Heat Exchangers

Due to the many benefits offered by Microchannel Heat Exchangers (MCHX), such as compactness, high heat transfer coefficients, reduced refrigerant charge, and energy and material cost savings, microchannel condensers and evaporators continue to be increasingly applied and investigated in the HVAC&R fields. One of the practical challenges associated with MCHX is the uniform distribution of two-phase refrigerant in the headers and tubes of the heat exchanger. In MCHX, which typically have port sizes about 1 mm or less, to maintain the pressure drop at reasonable levels while providing fairly uniform two phase flow distribution, an appropriate header size and number of tubes need to be chosen. In this paper, a critical review of experimental and analytical investigations of two-phase flow maldistribution in MCHX is presented. The influence of header and microchannel tube geometry, heat exchanger orientation, flow and operating conditions, fluid properties and flow patterns on the MCHX flow distribution is discussed. Researchers have investigated upward and/or downward two-phase flow in MCHX with horizontal and vertical headers, for which the microchannel tubes/ports are, respectively, vertical and horizontal. Traditionally, compared to investigations in horizontal headers, the studies on vertical headers have been relatively few. However, recently, due to applications involving automotive evaporators, more studies on vertical headers are reported. In all these studies, gravity is seen to profoundly affect the two-phase flow distribution. Various fluids such as R410A, R134a, R245fa, CO2, air-water, etc. have been studied in published works. Fluid thermophysical properties and flow patterns greatly influence the flow distribution in MCHX. Very few investigators have studied the effects of fluid properties on two-phase flow distribution. Zou and Hrnjak (2014)[16] speculated that fluids with high liquid to vapor density ratio would provide better flow distribution. However, this hypothesis needs to be experimentally confirmed.  Most studies agree that in headers, compared to annular flow, churn flow is desirable for better flow distribution. Most of the experimental investigations on flow distribution have been conducted for adiabatic flow. However, the applicability of such investigations to practical situations is dubious as the flow will be accompanied by condensation or boiling heat transfer. Refrigerant mass flux (G) and inlet quality (x) are seen to have a significant impact on flow distribution which is discussed in detail. Tube protrusion into the headers, and tube spacing also greatly affect the flow distribution. Since these interacting factors make the prediction of two-phase flow distribution very complex, a limited number of semi-empirical models/correlations have been proposed to quantify the two-phase flow maldistribution in MCHX. Five correlations for predicting the liquid take-off ratio in MCHX headers were assessed and among these five, Zou and Hrnjak (2013b)[15] correlation for R410A was found to perform reasonably. Based on the current study, recommendations regarding the applicability of these correlations to practical problems have been provided. Having identified and examined the key factors influencing two-phase flow maldistribution in MCHX, recommendations for further study are made.

To study the implications of Electronic Toll Collection System using RFID technology

Transportation is the backbone of any country’s economy. Due to increasing number of vehicles on the road, problems such as congestion, air pollution and many others have become a major factor of concern. Traditional toll collection booths require a number of operations like stopping the vehicle, lowering the window, finding the correct coinage or valid card before travelers can continue their journey. This research paper describes the Electronic toll collection system based on Radio Frequency Identification (RFID) technology. As time and efficiency are a matter of priority nowadays, the traditional method has to be reformed. In order to overcome the major issues of traffic congestion and time consumption RFID technology is used. RFID reader fixed at tollgate frame reads the tag attached to windshield of vehicle. The object detection sensor in the reader detects the approach of the incoming vehicle’s tag and toll deduction takes place through a prepaid card assigned to the concerned RFID tag that belongs to the owners’ account. Electronic toll collection system (ETC) has various advantages compared to traditional method

Efficacy of intravenous hydration drip versus amino acid drip in idiopathic oligohydramnios

Background: Role of intravenous hydration and amino acid infusion is well documented for the treatment of foetal growth restriction and associated oligohydramnios. The present study concentrates on the efficacy of intravenous hydration and amino acid infusions in cases of isolated oligohydramnios.Methods: 30 antenatal mothers having isolated oligohydramnios and fulfilling the inclusion criteria were selected for the study. They were randomized and administered 6 drips of normal saline or amino acid infusions. The AFI was assessed before and after the infusions. The rise in AFI, need of repeat infusions and foetal outcome was studied and analyzed.Results: The results were tabulated with respect to age of patients, gestational age, gravida status, AFI on admission and post infusions, comparison between the two groups after the 6 drips, need of repeat infusions and the maternal and perinatal outcome. Analysis of the data was performed using Chi-Square test.Conclusions: This study points towards the use of intravenous amino acids and normal saline in increasing the liquor in idiopathic oligohydramnios and proves useful in reducing perinatal morbidity and mortality and thus improving pregnancy outcomes, prolonging gestational age at time of delivery thereby improvement in weight gain. However the difference between the study groups was statistically not significant

Programmed labour compared with expectant management: is it truly a need of new millennium?

Background: The mechanism triggering the initiation of human parturition is still an enigma. At term a series of complex physiological, biochemical and physical processes cascade resulting in delivery of the fetus. This study deals exclusively with comparison of normal labour, induction of labour with prostaglandin, and with augmentation by intracervical insertion of PGE2 tablets, amniotomy and smooth muscle relaxant. Advantages and disadvantages of each of the above methods are compared with expectant management of labour. Aim of this study was to compare pros and cons of programmed labour that to with expectant management.Methods: Study was conducted in Department of Obstetrics and Gynaecology, Bharati Hospital and Research Centre, Pune. It was a prospective randomized clinical trial. 100 pregnant full term women, were selected for each group. At 0 hour primiprost tablet is inserted into the vagina close to the cervix. Frequency of repetition of tablet will be at three hours interval. Patient will be monitored.Results: The youngest one being of age 17 years and the eldest being of age 29 years. In this, we observed those primi and 2nd gravida patients 2-2 tablets each in latent phase and 1-1 tablets in active phase. The induction delivery Interval in primigravida was observed to be of average of 9 hours. While in II Gravida was 6.5 hours, in III Gravida 5.5 hours and in IV Gravida 4 hours.Conclusions: It has been proved beyond doubt that by programmed labour, the patient definitely can get the benefit of decrease in duration of labour

Ependymoma in pregnancy: one suspicion can save a life

Ependymomas arise from ependymal cells that line the ventricles of the brain and the spinal canal. Ependymomas are relatively rare tumors accounting for 2-3% of all primary brain tumors in adults. We present this case of ependymoma to highlight the fact that aggressive management might be keystone in saving mothers life and even after surgical intervention, it is possible that the tumour recurs and may prove fatal. Maternal outcome in primary brain tumours in pregnancy largely depend on histologic grade of primary tumour and gestational age at which they present

Electromagnetically induced transparency in cold 85Rb atoms trapped in the ground hyperfine F = 2 state

We report electromagnetically induced transparency (EIT) in cold 85Rb atoms, trapped in the lower hyperfine level F = 2, of the ground state 5$^{2}S_{1/2}$ (Tiwari V B \textit{et al} 2008 {\it Phys. Rev.} A {\bf 78} 063421). Two steady state $\Lambda$-type systems of hyperfine energy levels are investigated using probe transitions into the levels F$^{\prime}$ = 2 and F$^{\prime}$ = 3 of the excited state 5$^{2}P_{3/2}$ in the presence of coupling transitions F = 3 $\to$ F$^{\prime}$ = 2 and F = 3 $\to$ F$^{\prime}$ = 3, respectively. The effects of uncoupled magnetic sublevel transitions and coupling field's Rabi frequency on the EIT signal from these systems are studied using a simple theoretical model.Comment: 12 pages, 7 figure