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

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

Comparison of capillary based microflurometric assay for CD4+ T cell count estimation with dual platform Flow cytometry

The CD4+ T cell count estimation is an important monitoring tool for HIV disease progression and efficacy of anti-retroviral treatment (ART). Due to availability of ART at low cost in developing countries, quest for reliable cost effective alternative methods for CD4+ T cell count estimation has gained importance. A simple capillary-based microflurometric assay (EasyCD4 System, Guava Technology) was compared with the conventional flow cytometric assay for estimation of CD4+ T cell counts in 79 HIV infected individuals. CD4+ T cell count estimation by both the assays showed strong correlation (r = 0.938, p < 0.001, 95% CI 0.90 to 0.96). The Bland Altman plot analysis showed that the limits of variation were within agreeable limits of ± 2SD (-161 to 129 cells/mm(3)). The Easy CD4 assay showed 100% sensitivity for estimating the CD4+ T cell counts < 200 cells/mm(3 )and < 350 cells/mm(3 )and 97% sensitivity to estimate CD4+ T cell count < 500 cells/mm(3). The specificity ranged from 82 to 100%. The Kappa factor ranged from 0.735 for the CD4+ T cell counts < 350 cells/mm(3 )to 0.771 for < 500 cells/mm(3 )CD4+ T cell counts. The system works with a simple protocol, is easy to maintain and has low running cost. The system is compact and generates minimum amount of waste. Hence the EasyCD4 System could be applied for estimation of CD4+ T cell counts in resource poor settings

Towards a single step process to create high purity gold structures by electron beam induced deposition at room temperature

Highly pure metallic structures can be deposited by electron beam induced deposition and they have many important applications in different fields. The organo-metallic precursor is decomposed and deposited under the electron beam, and typically it is purified with post-irradiation in presence of O2. However, this approach limits the purification to the surface of the deposit. Therefore, 'in situ' purification during deposition using simultaneous flows of both O2 and precursor in parallel with two gas injector needles has been tested and verified. To simplify the practical arrangements, a special concentric nozzle has been designed allowing deposition and purification performed together in a single step. With this new device metallic structures with high purity can be obtained more easily, while there is no limit on the height of the structures within a practical time frame. In this work, we summarize the first results obtained for 'in situ' Au purification using this concentric nozzle, which is described in more detail, including flow simulations. The operational parameter space is explored in order to optimize the shape as well as the purity of the deposits, which are evaluated through scanning electron microscope and energy dispersive x-ray spectroscopy measurements, respectively. The observed variations are interpreted in relation to other variables, such as the deposition yield. The resistivity of purified lines is also measured, and the influence of additional post treatments as a last purification step is studied.EMPA is acknowledged for providing the original code for the GIS simulator model, which was extended by Stan de Muijnck (TU Delft) with the new geometry. Pleun Dona (FEI) is acknowledged for helping in the design of the concentric nozzle and in getting a working prototype. Patricia Peinado is also acknowledged for help on experimental activities. This work was supported by NanoNextNL program, a Dutch national research and technology program for micro- and nano-technology