Analysis of Electro-static Assisted Air Dehumidification Processes

The removal of water vapor from the air to reduce relative humidity is a well-known indoor environmental comfort requirement. Common dehumidification approaches require a substantial amount of energy and usually involve the cooling of atmospheric humid air below its dew point or the use of absorbent/adsorbent materials to extract water vapor out of the air. More recently, researchers investigated the effect of electrostatic forces for enhancing water vapor condensation. However, the studies are limited, and there is a lack of correlations that can predict the dehumidification rate. Also, the findings from the literature focused mainly on small flow rates on the order of one to two cfm. Electrically-enhanced condensation consists of the use of highly charged particles, preferably highly charged water droplets, that attract polar water vapor molecules to their surfaces and promote condensation, a phenomenon known as dielectrophoresis. An effect of the electric charge is the reduction of the vapor pressure on the droplets\u27 surface with respect to the saturated pressure predicted by the Kelvin equation. Consequently, the equilibrium between evaporation and condensation is shifted towards condensation. Following the application of the modified Kelvin-Thomson theory, we developed a preliminary physics-based model to predict an effective size range of the charged droplets for optimal dehumidification. The range resulted in about 2 to 4 μm in diameter, under few simplifying assumptions. The effect of the size and the charge of the electrosprayed droplets on the overall dehumidification rate was briefly discussed. The use of electrosprays to produce small but highly charged droplets was broadly discussed in this paper. The cone-jet mode was identified as the most suitable electrospray operational mode, and it generated droplets of small size and high electrical charge. The cone-jet stability was also analyzed in detail. The preliminary data of the present work and the model results indicated that several electrospray heads were required to achieve a 5% dehumidification rate for airflow rates of about 5 cfm

Two Phase Flow Boiling Heat Transfer and Pressure Drop of Two New LGWP Developmental Refrigerants Alternative to R-410A

To meet the terms of the Montreal Protocol, CFCs and HCFCs have been gradually phased out and they have been replaced by refrigerants that have zero ozone depletion potential. However, some of these fluids, such as R-410A, have global warming potential (GWP) that might still be of concern from an environmental perspective in case of leakage or improper charge management. Few studies of refrigerants that have zero ozone depletion potential and GWP less than 500 are available in the literature. Preliminary findings from these studies suggested that new development refrigerants were viable options. System COP and capacity were promising but there is not much information on the heat transfer and pressure drop characteristics of these low GWP refrigerants. This paper contributes to address this gap and provides new data for the two phase flow boiling heat transfer coefficient (HTC) and pressure drop of two new low GWP developmental refrigerant alternatives for R-410A. Heat transfer measurements were conducted for a copper tube commonly used in direct expansion evaporators of air conditioning systems with a 9.5 mm (0.375 in.) outside diameter and internally enhanced micro-finned surface. Data of local two phase flow HTC and pressure drop are presented for refrigerants R-410A, R-32, R-1234yf and the two new developmental refrigerants referred to as DR-5 and DR-5A. The experimental findings from this work indicated that the refrigerant R-32 had similar and slightly higher heat transfer coefficient than that of R-410A at same refrigerant mass flux and similar heat flux conditions on the outer surface of the tube. Refrigerant R-1234yf had about 15 to 20% lower heat transfer coefficient than R410-A at 4?C saturation temperature. For this saturation temperature the developmental refrigerants DR-5 and DR-5A had heat transfer coefficients between R-32 and R-1234yf when the vapor quality ranged from 0.2 to 0.7. An increase of the saturation temperature from 4°C to 9°C decreased the heat transfer coefficients for all of the refrigerants tested. The two phase flow boiling pressure drops increased monotonically if the vapor quality of the refrigerant increased. The pressure drops of refrigerant R-410A were the lowest while the pressure drop for refrigerant R-1234yf were the highest measured among the fluids investigated. The developmental refrigerants DR-5 and DR-5A showed identical characteristics in terms of pressure drop at both saturation temperatures of 4°C and 9°C

Thermal Performance and Moisture Accumulation of Mechanical Pipe Insulation Systems Operating at Below Ambient Temperature in Wet Conditions with Moisture Ingress

When pipes are used for chilled water, glycol brines, refrigerants, and other chilled fluids, energy must be spent to compensate for heat gains through the wall of the pipes. Higher fluid temperature at the point of use decreases the efficiency of the end-use heat exchangers and increases the parasitic energy consumption. Mechanical pipe insulation systems are often used to limit the heat gains and save energy in commercial buildings. Pipe insulation systems play an important role for the health of the occupied space. When a chilled pipe is uninsulated or inadequately insulated, condensation might occur and water will drip onto other building surfaces possibly causing mold growth. The critical issue with cold pipes is that the temperature difference between the pipe and its surrounding ambient air drives water vapor inside the insulation system and condensation commonly occurs when the water vapor comes in contact the chilled pipe surface. This paper experimentally studies this issue for pipe insulation systems operating at below ambient temperature. The moisture content and the associated thermal conductivity of several pipe insulation systems were measured under various wet condensing conditions with moisture ingress. Accelerated type tests in laboratory highlighted the propensity of moisture accumulation in the insulation systems with cylindrical configuration and with split longitudinal joints. The moisture accumulation rate was measured and the apparent thermal conductivity increased significantly in a 60 days period when water vapor entered a pipe insulation syste

Experimental and Theoretical Investigation of Oil Retention in Vapor Compression Systems

The design of any system needs to consider a number of parameters according different needs. In heating, ventilation and air conditioning systems the overall efficiency, the reliability of the components, the cost and volume, and the refrigerant/oil charge are only some examples of variables that can be optimized. An important aspect is the selection of lubricants that provide the same or improved characteristics relative to traditional mineral oils. In HVAC systems, the oil exists only because the compressor requires it for lubrication and sealing. Proper oil management is necessary in order to improve the compressor reliability, increase the overall efficiency of the system, and minimize the system cost by avoiding redundancy. Several literature sources focused on oil/refrigerant properties (Thome, 1995), oil return characteristics (Biancardi et al., 1996) and oil transport phenomena (Mehendale, 1998). An analytical and experimental study of the oil retention has been developed for automotive air conditioning systems using carbon dioxide (Jun-Pyo Lee, 2002). However, a general comprehensive model for oil retention and oil distribution in heat pump systems using other refrigerant/oil mixtures does not exist and is of importance to future design considerations. The purpose of this thesis is to experimentally and theoretically investigate the physics of oil retention and oil transport in different components of the system. Condenser, evaporator, suction and liquid lines are studied using different pairs of refrigerant-oil mixtures. Oil retention is measured directly using an experimental apparatus, and oil film thickness is estimated. At oil mass fractions of 8 wt.%, the pressure drops increase up to 40% in the suction line, 20% in the evaporator and 30% in the condenser as compared to oil-free operating conditions. New pressure drop correlations need to include this penalty factor due to oil retention. An analytical model for vapor and two-phase refrigerant flows utilizing minimal empirical data is developed. The model is able to estimate the oil distribution in the entire system providing good design guidelines for the selection of the proper refrigerant/oil mixture, the optimization of the component geometries, and the management of the oil/refrigerant charge

A Comparison Between Recent Experimental Results and Existing Correlations for Microfin Tubes for Refrigerant and Nanolubricants Mixtures Two Phase Flow Boiling

Driven by higher energy efficiency targets, there is critical need for major heat transfer enhancements in heat exchangers. Nanolubricants, that is, nanoparticles dispersed in the non-volatile component of a mixture, have the potential to increase the heat transfer coefficient by 20% or more for two-phase flow boiling with small or no penalization on the two-phase flow pressure drop. The present work builds upon these intriguing yet unexplained findings, which were documented in the experiments of the present study for one type of nanolubricant, but for which a theory still does not exist. This paper presents a comparison between existing models in the literature and recent new experimental data for two phase flow boiling in a microfin tube of refrigerant R410A and nanolubricants mixtures. Alumina Oxide (g-Al2O3) based nanolubricants with 40 nominal particle diameter of approximately spherical shape were investigated. The nanoparticles concentration in the lubricant varied from 10 to about 20 in mass percentage, and the lubricant concentration varied from 0 up to 3% in mass percentage. The models available in the open domain literature were not able to capture the effects of the nanoparticles on the two-phase flow heat transfer coefficient. The augmented thermal conductivity of the lubricant due to the addition of highly conductive nanoparticles was not the main mechanism responsible for the heat transfer enhancements. The discrepancy between the simulation results and the experimental data was postulated to be due to non-Newtonian behaviors due to the presence of nanoparticles and surfactants. The flow development of the liquid phase of the mixture and the localized thickening and thinning of the liquid film thickness around the inner walls of the tube can alter the film local convective thermal resistance.

Effect of Surface Wettability on Droplets Growth during Water Condensation and Initial Droplets Icing

Air-source heat pump systems extract heat directly from the cold outdoor ambient and reject heat to the warm indoor environments of residential and commercial buildings. During their winter operation, the outdoor coil often accumulates frost on its surface. Frost acts as an insulator and blocks air passages, reducing the heat transfer rate and increasing the pressure drop of air passing through the coil. Defrost cycles are periodically executed between the heating times to melt the ice, drain the water from the outdoor coil, and free accumulated frost before the heating service can start again. Unfortunately, too many defrost cycles penalize the efficiency of the heat pumps. Most research in frost mitigation focused on superhydrophobic surfaces, lubricant impregnated surfaces, and nanostructured surfaces. Some studies proposed surface types that would lower ice adhesion such that droplet removal was promoted before freezing. However, the mitigation effects of these surfaces can be sensitive to experimental conditions and surface structure. Additionally, in circumstances where frost formation cannot be prevented due to the operating conditions, the challenge of predicting frost nucleation and growth rate is further complicated by transient flow conditions with combined heat and mass transfer phenomena to moving frost boundaries. This paper presents new data of freezing time, droplet diameter, and droplet shape with different surface wettability during initial droplet icing. Water condensation and icing formed on the flat plates for convective channel flows. Four surfaces with different wettability were investigated under two test conditions. The contact angle ranged from less than 10 degrees (i.e., superhydrophilic) to over 109 degrees (i.e., hydrophobic). Two surfaces shared similar contact angles but had different coating components. Because frost nucleation was partially a stochastic phenomenon subjected to many variables that were difficult to control and replicate even in a laboratory setting, frost tests with identical environmental and surface temperature conditions were repeated several times to gather meaningful averages for the freezing time and to quantify the magnitude of potential variability in the frost nucleation time and droplets size due to the surface wettability characteristics. The new data presented in this paper are used to inform and validate physics-based frost models, predicting the nucleation features and actual frost formation time for coated fin structures of heat exchangers