Experimental and Theoretical Investigation of Effect of Fin Geometry on Frost Formation on Microchannel Heat Exchangers

This dissertation focuses on various aspects of frost growth on louvered folded fins in outdoor microchannel heat exchangers used in air source heat pump systems. In this research, the effects of surface temperature, fin geometries, surface coatings and environmental parameters such as air humidity and velocity on the thermal and hydraulic performance of the microchannel heat exchangers under frosting condition were investigated. The overall aim was to provide some guidelines about frost behavior of microchannel heat exchangers and isolate and quantify the effect of geometry, surface coatings and surface temperature. A set of empirical correlations were developed to predict frost thickness, air face velocity drop and heat transfer coefficients for various fin geometries. The approach taken was to perform laboratory experiments on small scale coils and sample fins that were cut out from commercially available heat exchangers and to replicate their operating conditions in laboratory. A good estimation of the fin surface temperature was achieved with the methodology developed in the present study. Experimental data of frost weight, local frost thickness, air pressure drop across the coils, time of frost-defrost cycles and heat transfer rates were recorded for heat exchangers operating in actual transient frosting conditions. Data showed that the frosting time and the frost growth rates depend mainly on the local fin surface temperature while water retention and surface coatings could have secondary and minor effects. Some geometries performed better in frosting condition than others such as geometries with lower fin density that delayed the blockage of the air flow. The fin length and fin depth had minor effects on frosting performance. It was found that air humidity has a significant effect on rate of frost formation while air velocity seemed to have a small effect on frost formation.Mechanical & Aerospace Engineerin

Frosting Performance of Fin-and-Tube Evaporators with Small Copper Tubes Diameter

In modern heat pump systems, the heat exchangers use enhanced heat transfer surfaces for both air and refrigerant sides. In air conditioning applications, conventional 9.5 mm (3/8 inch) tube diameter fin-and-tube coils are slowly being replaced by microchannel heat exchangers. However, during heating mode the energy performance of heat pump systems with microchannel outdoor coils are generally lower than those of fin-and-tube direct-expansion evaporators due to a higher frequency of defrost cycles. A different approach might be to utilize fin-and-tube technology, which has proven records of excellent water drainage characteristics and good performance in frosting operating conditions, and enhance the air side heat transfer rates by introducing a larger number of small diameter copper tubes. In this paper, six fin-and-tube coils with copper tube diameter ranging from 5 mm (1/5 inch) to 7mm (8/29 inch) were experimentally investigated in frosting operating conditions. The laboratory experiments were conducted in an air flow wind tunnel at Oklahoma State University. Experimental data on heat transfer rate and air-side pressure drop across the coils were measured and the fin density and the tube diameter were varied in a parametric fashion during the experimental campaign. The performance of the fin-and-tube coils were also compared to those of a conventional 9.5 mm copper tube diameter fin-and-tube heat exchanger and of a microchannel heat exchanger that had similar air-side frontal area and at similar operating conditions of outdoor direct-expansion evaporators in heat pump systems for residential applications. The trends of the data during frosting operation suggested that reducing the tube diameter was beneficial for frosting performance at low fin density while was harmful at high fin density. The data showed that increasing the fin density increased the capacity but significantly reduced the time for heating service for the evaporator. Small copper tube diameter resulted in about 11% higher initial capacity at dry start conditions and about 4% higher average integrated capacity when considering the entire frost period. The data discussed in this paper serve as basis for future research on direct-expansion evaporators for air-source heat pump applications, in which the frosting of the outdoor heat exchangers is still one of the major concerns

Biomaterials in Valvular Heart Diseases

Valvular heart disease (VHD) occurs as the result of valvular malfunction, which can greatly reduce patient\u27s quality of life and if left untreated may lead to death. Different treatment regiments are available for management of this defect, which can be helpful in reducing the symptoms. The global commitment to reduce VHD-related mortality rates has enhanced the need for new therapeutic approaches. During the past decade, development of innovative pharmacological and surgical approaches have dramatically improved the quality of life for VHD patients, yet the search for low cost, more effective, and less invasive approaches is ongoing. The gold standard approach for VHD management is to replace or repair the injured valvular tissue with natural or synthetic biomaterials. Application of these biomaterials for cardiac valve regeneration and repair holds a great promise for treatment of this type of heart disease. The focus of the present review is the current use of different types of biomaterials in treatment of valvular heart diseases

New Correlations for the Air-Side Heat Transfer Coefficient of Microchannel Heat Exchangers Under Quasi-Steady State Frosting Operating Conditions

This study experimentally investigated the frost growth on louvered folded fins in outdoor microchannel heat exchangers used in air-source heat pump systems. The effects of surface temperature and fin geometries on the performance of the microchannel heat exchangers under frosting condition were studied. 7 fin samples with various fin width, fin height and fin density were tested in controlled laboratory conditions that replicated those of actual heat pump systems in winter season. The fin surface temperature was experimentally estimated with the novel methodology developed in the present study. Experimental data of local frost thickness, air pressure drop across the coils, time of frost-defrost cycles and heat transfer rates were recorded for heat exchangers operating in actual transient frosting conditions. Data showed that the frosting time and the frost growth rates were depended mainly on the local fin surface temperature. A set of empirical correlations were developed to predict the frost thickness on fin leading edge and the reduction of air face velocity due to air pressure drop across the frosted coil during frosting operation. The correlations aid to calculate the instantaneous air-side Reynolds numbers during frosting operation of the fin samples. These are critical for predicting the heat transfer rates of the microchannel coils in quasi-steady state frosting operating conditions

Additional file 1 of Evaluation of the primary health care expansion program with public-private partnership in slum areas from the perspective of stakeholders: a qualitative study

Supplementary Material 1

Ethics of research on stem cells and regenerative medicine: ethical guidelines in the Islamic Republic of Iran

BackgroundRegenerative medicine plays a major role in biomedicine, and given the ever-expanding boundaries of this knowledge, numerous ethical considerations have been raised.Main textRapid advancement of regenerative medicine science and technology in Iran, emerged the Iranian National Committee for Ethics in Biomedical Research to develop a comprehensive national ethical guideline. Therefore, the present ethical guideline which comprises eleven chapters was developed in 2019 and approved in early 2020. The titles of these chapters were selected based on the ethical considerations of various aspects of the field of regenerative medicine: (1) ethical principles of research on stem cells and regenerative medicine; (2) ethical considerations for research on stem cells (embryonic stem cells, epiblast stem cells, tissue-specific stem cells, stem cells derived from transdifferentiation, induced pluripotent stem cells [iPSCs], germline pluripotent stem cells, germline stem cells, and somatic cell nuclear transfer [SCNT] stem cells); (3) ethical considerations for research on somatic cells in regenerative medicine (adult somatic cells, fetal tissue somatic cells, and somatic cells derived from pregnancy products [other than fetus]); (4) ethical considerations for research on gametes in regenerative medicine; (5) ethical considerations for research related to genetic manipulation (human and animal) in regenerative medicine; (6) ethical considerations for research on tissue engineering in regenerative medicine; (7) ethical considerations for pre-clinical studies in regenerative medicine; (8) ethical considerations for clinical trials in regenerative medicine; (9) ethical considerations for stem cells and regenerative medicine bio-banks; (10) ethical considerations for privacy and confidentiality; and (11) ethical considerations for obtaining informed consent.ConclusionThis article discusses the process of developing the present ethical guidelines and its practical points. We hope that it can play an important worldwide role in advancing ethics of research on stem cells and regenerative medicine