Vapors

This paper reviews on the possibilities of researches in the field of exergy analysis in various usable sectors where vapor compression refrigeration systems are used. Here, it is found that exergy depends on evaporating temperature, condensing temperature, sub-cooling and compressor pressure. It also depends on environmental temperature. Nowadays, hydrocarbons are considered as refrigerant having low ODP and GWP, and these are considerable in the aspect of exergy analysis. Refrigerants R 407a, R 600a, R 410a and R 134a are considered and analyzed with respect to exergy efficiency. Mixtures of hydrocarbons with R134a also show better performance with respect to other refrigerants. Among the components of the vapor compression system, much research showed that major part of exergy losses is occurred in the compressor. Nanofluid and nanolubricant cause to reduce the exergy losses in the compressor indirectly. © 2010 Elsevier Ltd.

http://www.sciencedirect.com/science/article/pii/S1364032110004089 
http://ac.els-cdn.com/S1364032110004089/1-s2.0-S1364032110004089-main.pdf?_tid=bc1fc33c-a9c7-11e3-ae54-00000aacb360&acdnat=1394616287_221e1c9cd933c24fe68a0834a938ef2d

This paper emphasized on the possibilities of researches in the field of exergy analysis in various usable sectors where vapor compression refrigeration systems are used. Exergy losses, exergy efficiency, second law efficiency and irreversibility of the system components as well as of the whole system are measured. In the vapor compression system, R134a, R290 and R600a are considered as refrigerants. Exergy parameters in the compressor, evaporator, condenser and expansion devices are calculated and analyzed. Exergy losses depend on evaporator temperatures, condensing temperature, refrigerants and ambient temperature. Most of the exergy losses occur in the condenser. Expansion device has the lowest losses. Exergy parameters are compared for different operating temperature. It is found that hydrocarbons (R600a) have 50% higher exergy efficiency than R134a. Mixture of hydrocarbons also shows the best performance based on the exergy analysis. © 2011 IEEE.

http://ieeexplore.ieee.org/ielx5/6032030/6041433/06041480.pdf?tp=&arnumber=6041480&isnumber=6041433

An experimental investigation is presented concerning the boiling of water at atmospheric pressure on a single copper surface in which the spatial distribution of active nucleation sites was investigated at different levels of heat flux and subcooling. The results obtained ...

The main motivation of conducting this work is to present a rigorous analysis and investigation of the potential effect of the heat pipe adiabatic region on the flow and heat transfer performance of a heat pipe under varying evaporator and condenser conditions. A two-dimensional steady-state model for a cylindrical heat pipe coupling, for both regions, is presented, where the flow of the fluid in the porous structure is described by Darcy–Brinkman–Forchheimer model which accounts for the boundary and inertial effects. The model is solved numerically by using the finite volumes method, and a fortran code was developed to solve the system of equations obtained. The results show that a phase change can occur in the adiabatic region due to temperature gradient created in the porous structure as the heat input increases and the heat pipe boundary conditions change. A recirculation zone may be created at the condenser end section. The effect of the heat transfer rate on the vapor radial v...

This experimental and analytical study focuses on the formation of banded condensate films on a horizontal tube, when the ammonia mass fraction of the vapor is less than 0.0126. The experimental results show that there is a step change in the heat transfer coefficient when a banded film is formed. This change develops when the vapor mass fraction is approximately 0.004 and the heat transfer coefficient at the end of the change is 15% greater than the value expected for steam. The banded films are initiated at the base of the tube; midway between the regions where droplets detach. Photographs taken during the tests show that the development starts with the formation of a single band; when this band is complete, secondary bands are formed on either side and then further bands are added until the complete tube surface is covered with a banded film. The reason for the delay in the formation of the bands is examined using a simple two-dimensional force balance. The success of this examin...

Due to the increase in energy prices and spiralling consumption, there is a need to greatly reduce the cost of electricity within data centers, where it makes up to 50% of the total cost of the IT infrastructure. A technological solution to this is using on-chip cooling with a single-phase or evaporating liquid to replace energy intensive air-cooling. The energy carried away by the liquid or vapor can also potentially be used in district heating, as an example. Thus, the important issue here is “what is the most energy efficient heat removal process?” As an answer, this paper presents a direct comparison of single-phase water, a 50% water–ethylene glycol mixture and several two-phase refrigerants, including the new fourth generation refrigerants HFO1234yf and HFO1234ze. Two-phase cooling using HFC134a had an average junction temperature from 9 to 15 °C lower than for single-phase cooling, while the required pumping power for the central processing unit cooling element for single-pha...

Hydrocarbons being natural fluid have drawn much attention of the scientists and researchers for the application as a sustainable material for the vapor compression refrigeration system. This paper presents a comparison of the energetic and exergetic performances of a domestic refrigerator using pure butane and isobutane as refrigerants. The thermodynamic performances such as exergy destruction or losses, exergy efficiency, and coefficient of performances (COP) were investigated. These parameters were measured at varied operating conditions. Exergy and energy efficiencies of isobutane were found to be 50% and 175% higher than that of R-134a. The analysis shows that the performances of butane and isobutane as refrigerants are comparable with HFC134a. It has also been found that at higher evaporating temperatures, the exergy losses are minimal. The maximum exergy loss occurred in the compressor and the value was 69% of the whole losses in the system. Highest sustainability index was found for butane compared to that of R134a and R600a, respectively. Copyright © Taylor & Francis Group, LLC.

http://www.tandfonline.com/doi/pdf/10.1080/15435075.2011.621491

Microfluidics allows for the miniaturization of laboratory processes onto a compact disc (CD). A microfluidic CD provides a cost-effective, portable and automated diagnostic platform without the use of bulky equipment and complex machinery. This reduction in the cost, footprint, and user input allows for the development of tools suitable for point-of-care applications. One of the criteria for point-of-care applications is the ability to have long storage life. During storage, the reagents/chemicals in typical microfluidic CDs might evaporate and mix together before actual usage, compromising on the integrity of the result. To complement the use of capillary valves, this paper presents various available valves for vapor control, and introduces a hybrid capillary-flap valve for use in microfluidic CDs for point-of-care applications. © 2011 Springer-Verlag.

http://link.springer.com/chapter/10.1007/978-3-642-21729-6_144

Abstract : Computer programs for data collection and analysis of interferograms from a commercial Fourier Transform Infrared sensor were developed. Programs written in C language for an IBM PC using the DOS operating system allow one to collect and display data in a variety of formats useful for the environmental monitoring of vapor clouds. Software is described that enables the user to execute signal processing algorithms for the real-time analysis of interferograms. The programs collect data from the commercial interferometer and detect a vapor species using digital filters and pattern recognition methods. Source code and documentation describing all program functions are provided. Fourier Transform Infrared, Infrared, Infrared background suppression, FTIR, Passive infrared software

Abstract : The grant as in support of a research program to explore the possibility of a microwave atomic clock based on an atomic fountain of cesium atoms. During the two year granting period, the following work was done: A large magneto-optic trap in a vapor cell was constructed where over 4x 10(exp 10) atoms were trapped at a density on the order of 10(exp 11) atoms/cu cm. This work increased the number of atoms optically trapped by a factor of -400 compared to previous work. The optical density of the atoms in this trap has a peak attenuation of e(-175). In an invited review article we examined the prospects for slow atom frequency standards. Most of the systematic effects that limit the current accuracy today's frequency standards decrease as the velocity of the atoms is reduced. We identified the one possible exception: collisions between ultra-cold atom in an atomic fountain could induce a phase shift that would ultimately limit the accuracy of the fountain clock. We brought the problem to the attention of B. Verhaar, who began a series of calculations on the effects of low energy collisions. jg p.4

The usefulness of a tent used in cold weather depends upon, among other things, the condensation of water vapor on the tent walls as this condensate can wet occupants' insulating garments, increase the tent weight and increase the tent's packed volume. In this report, the heat transfer and moisture transport across a single fabric layer as might occur in a tent is studied by means of mathematical models. The effects of water-vapour permeability of the fabric, interior air temperature and relative humidity and the ambient temperature on the condensation rate within the tent are examined. It was found that small decreases in the water-vapour permeability of the fabric layer can result in large increases in the condensation rate at the wall. Reducing the interior tent temperature or relative humidity can significantly reduce the condensation rate. The results indicate that a substantial portion of the water vapor within a very humid tent atmosphere escapes through the tent wall...

The sensing of chemicals and biochemical molecules using several porous silicon optical microsensors, based both on single-layer interferometers and resonant-cavity-enhanced microstructures, is reported. The operation of both families of sensors is based on the variation of the average refractive index of the porous silicon region, due to the interaction with chemical substances either in vapor or liquid state, which results in marked shifts of the device reflectivity spectra. The well established single-layer configuration has been used to test a new chemical approach based on Si-C bonds for covalent immobilization of biological molecules, as probe, in a stable way on the porous silicon surface. Preliminary results on complementary oligonucleotide recognition, based on this technique, are also presented and discussed. Porous silicon optical microcavities, based on multilayered resonating structures, have been used to detect chemical substances and, in particular, flammable and toxic organic solvents, and some hydrocarbons. The results put in evidence the high sensitivity, the reusability, and the low response time of the resonant-cavity-enhanced sensing technique. The possibility of operating at room temperature, of remote interrogation, and the absence of electrical contacts are further advantages characterizing the sensing technique.

... films for gas-sensing applications Biju Philip, Jose K Abraham, Anupama Chandrasekhar and Vijay K Varadan Center for Engineering of Electronics and Acoustics Materials and Devices, Pennsylvania State University, University Park, PA 16802, USA ...

A boiling liquid, expanding vapor explosion (BLEVE) occurs when a pressure vessel containing a superheated liquid undergoes a catastrophic failure, resulting in a violent vaporization of the liquid. The exposure of a pressure vessel to a fire is a classic scenario that can result in a BLEVE. The thermomechanical exergy of a pressure vessel’s contents provides — by definition — an upper bound on the work that can be performed by the system during the explosion. By fixing the values of ambient pressure and temperature (i.e., the dead state), exergy can be interpreted as another thermodynamic property. This rigorous and unambiguous definition makes it ideal to estimate the maximum energy of explosions. The numerical value of exergy depends on the definition of the dead state. In this paper we examine the effect of different definitions for the dead state on the explosion energy value. We consider two applications of this method: the contribution of the vapor head-space to the explosive...

This paper addresses the numerical simulation of two-phase flow heat transfer in the helically coiled tubes of an integral type pressurized water reactor steam generator under normal operation using a computational fluid dynamics code. The shell-side flow field where a single-phase fluid flows in the downward direction is also calculated in conjunction with the tube-side two-phase flow characteristics. For the calculation of tube-side two-phase flow, the inhomogeneous two-fluid model is used. Both the Rensselaer Polytechnic Institute wall boiling model and the bulk boiling model are implemented for the numerical simulations of boiling-induced two-phase flow in a vertical straight pipe and channel, and the computed results are compared with the available measured data. The conjugate heat transfer analysis method is employed to calculate the conduction in the tube wall with finite thickness and the convections in the internal and external fluids simultaneously so as to match the fluid...