Steven J. Eckels

Local Heat Transfer Coefficients during Condensation of R-22 and R-32/R-125 Mixtures

In this paper local and average heat transfer coefficients during condensation of refrigerants R-22 and two mixtures of R-32/R-125 (45%/55% and 25%/75%) are presented. The local heat transfer coefficients were determined using a specially instrumented test section that measured local energy flow and surface temperatures during condensation. The local test section used an 8.0 mm (0.32 in) inner diameter smooth tube that was 1.93 m (6.3 ft) long. Average heat transfer coefficients were determined using the Log-Mean-Temperature-Difference equations. The local method yielded useful information on the variation of heat transfer coefficients with refrigerant quality and mass flux. Typically, heat transfer coefficients were found to double from low to high quality at a particular mass flux, while smaller increases occurred with mass flux. Evaluation of local heat transfer coefficients confirmed that the method yields reasonable results. Specifically, comparison of the average of local heat transfer coefficients ...

An Experimental Investigation of In-Tube Evaporation of Pure Ammonia in a Smooth and a Microfin Tube, Part I—Heat Transfer (RP-866)

Average and sectional-average heat transfer coefficients were measured for the evaporation of pure ammonia inside horizontal smooth and microfin tubes. The test sections were tube-in-tube, counterflow heat exchangers with ammonia flowing in the inner tube and liquid R-134a in the annulus. Both the smooth and the microfin test sections were constructed with a 3.048 m (10.0 ft) long aluminum tube placed inside a 19.0 mm (0.75 in.) outside diameter (OD) outer tube. Data were collected at mass fluxes of 9, 27, 47 and 61 kg/(m2·s) (6600, 20,000, 35,000 and 45,000 lb/h·ft2), saturation temperatures of 5, −10 and −20°C (41, 14 and −4°F) and heat fluxes of 820, 2710, and 5430 W/m2 (260, 860, and 1720 Btu/h·ft2). Average heat transfer coefficients were measured for a 15 to 95% quality range at each saturation temperature for all mass fluxes. Sectional-average heat transfer coefficients were measured for each saturation temperature and heat flux at nominal test-section vapor qualities of 10, 25, 50, 75, and 95%. The improved surface-wetting characteristics of the microfin tube were found to increase the heat transfer coefficient relative to that measured in the smooth tube. This effect was greatest at low mass fluxes when the fluid flow was laminar and stratified. A comparison of the experimental data with current correlations available for evaporation showed that no correlation accurately predicted the heat transfer coefficients over the entire range of conditions.

Single-Phase Flow in Meso-Channel Compact Heat Exchangers for Air Conditioning Applications

Experimental study of the single-phase heat transfer and fluid flow in meso-channels, i.e., between micro-channels and mini-channels, has received continued interest in recent years. The studies have resulted in empirical correlations for various geometries ranging from simple circular pipes to complicated enhanced noncircular channels. However, it is still unclear whether the correlations developed for conventional macro-channels are directly applicable for use in micro-/mini-channels, i.e., hydraulic diameter less than 3 mm, with heat exchanger applications. A few researchers have agreed that similar results may be obtained for the laminar flow regime regardless of the channel size, but no general agreement has been reached for the transitional and turbulent flow regimes yet. In this study, different meso-channel air–liquid compact heat exchangers were evaluated and the experimental results were compared with published empirical correlations. A modified Wilson plot technique was applied to obtain the heat transfer coefficients, and the Fanning equation was used to calculate the pressure drop friction factors. The uncertainty estimates for the measured and calculated parameters were also calculated. The results of this study showed that the well-established heat transfer and pressure drop correlations for the macro-channels are not directly applicable for use in the compact heat exchangers with meso-channels.

An Investigation of Condensation Heat Transfer Performance of HFC-134a on Single Enhanced Tubes (RP-984)

Tests to measure shell-side heat transfer coefficients were performed on a single-tube test facility. HFC-134a was condensed on the horizontal test tube with cold water working as the coolant. Two 19.1 mm (0.75 in.) outer diameter (OD) enhanced tubes, one a two-dimensional (2D) finned tube and the other a three-dimensional (3D) enhanced tube, were used to construct the test section. For comparison purposes, a smooth tube of the same nominal diameter was also tested. The experiments were conducted over a heat flux range of 3155 to 63 100 W/m2 (1000 to 20,000 Btu/h·ft2) at saturation temperatures of 40 and 45°C (104 and 113°F). The tests revealed that the shell-side heat transfer coefficient dropped as heat flux increased. The enhanced tubes significantly improved the condensation heat transfer effectiveness compared with the smooth tube, with the 3D enhanced tube having the highest heat transfer coefficients. Specifically, the 2D tube had heat transfer coefficients that were 8.0 times better than the smooth tube, whereas enhancements of 11.8 times were seen in the 3D tube. The heat transfer coefficients for each tube were also compared with existing single-tube models.

Update of the scientific evidence for specifying lower limit relative humidity levels for comfort, health, and indoor environmental quality in occupied spaces (RP-1630)

Nearly 600 articles were located in citation and keyword searches regarding the effects of humidity on comfort, health, and indoor environmental quality. Of these, around 70 articles reported the effects of low humidity (relative humidity ≤ 40%) and were analyzed in detail. Information in some categories was well chronicled, while other categories had significant knowledge gaps. Low humidity decreased house dust mite allergens. Due to different envelopes, generalizations could not be made for all bacteria and viruses. However, lower humidity increased virus survival for influenza. For comfort, low humidity had little effect on thermal comfort, but skin dryness, eye irritation, and static electricity increased as humidity decreased. For indoor environmental quality, low humidity had nonuniform effects on volatile organic compound emissions and perceived indoor air quality. Across many low humidity studies, ventilation rates and exposure times were noted as confounding variables. A majority of studies that used human subjects utilized exposure times of 3 h or less with adult subjects; few studies used children, adolescents, or elderly subjects.

Aircraft recirculation filter for air quality and incident assessment

The current research examines the possibility of using recirculation filters from aircraft to document the nature of air-quality incidents on aircraft. These filters are highly effective at collecting solid and liquid particulates. Identification of engine oil contaminants arriving through the bleed air system on the filter was chosen as the initial focus. A two-step study was undertaken. First, a compressor/bleed air simulator was developed to simulate an engine oil leak, and samples were analyzed with gas chromatograph-mass spectrometry. These samples provided a concrete link between tricresyl phosphates and a homologous series of synthetic pentaerythritol esters from oil and contaminants found on the sample paper. The second step was to test 184 used aircraft filters with the same gas chromatograph-mass spectrometry system; of that total, 107 were standard filters, and 77 were nonstandard. Four of the standard filters had both markers for oil, with the homologous series synthetic pentaerythritol esters being the less common marker. It was also found that 90% of the filters had some detectable level of tricresyl phosphates. Of the 77 nonstandard filters, 30 had both markers for oil, a significantly higher percent than the standard filters.

Analysis of particulate size distribution and concentrations from simulated jet engine bleed air incidents

Engine oil migrating into the bleed air stream of aircraft environmental control systems occurs with enough frequency and deleterious effects to generate significant public interest. While previous work has explored the chemical makeup of the contaminants in the aircraft cabin during these events, little is known about the characteristics of the aerosol resulting from oil contamination of bleed air. This article presents particle counter data (giving both size distributions and concentration information) of the oil droplets from simulated jet engine bleed air. Four particle counters—a scanning mobility analyzer, an aerodynamic particle-sizer, an optical particle counter, and a water-based condensation particle counter—were used in the study encompassing a size range from 13 nm to 20 μm. The aerosol characterization is given for different bleed air temperatures and pressures. The data show a substantial increase of ultrafine particles as the temperature is increased to the maximum temperatures expected during normal aircraft operation. This increase in ultrafine particles is consistent with smoke generated from the oil. The pressure of the bleed air had little discernible effect on the particle size and concentration.

Effect of Inundation Upon the Condensation Heat Transfer Performance of R-134a: Part II—Results (RP-984)

Shell-side heat transfer coefficients were experimentally determined for three different types of tubes in a 25-tube bundle designed to simulate a single-pass heat exchanger. Smooth tubes, two-dimensional integral fin tubes, and three-dimensional enhanced-surface tubes were arranged in a three-column-by-ten-row staggered pattern with a horizontal and vertical pitch of 22.9 mm. Bundle average heat transfer coefficients and row-by-row heat transfer coefficients were measured as part of this study. Results showed the three-dimensional enhanced tube had the highest condensation heat transfer coefficient of any tube tested during the course of the study. Bundle average heat transfer coefficients for the three-dimensional tube showed a steady decline with increased heat flux and the row-by-row heat transfer coefficients showed an almost linear decrease with increased row number. The performance of the two-dimensional tube showed variations with heat flux but showed less sensitivity to inundation level. The experimental results for all tubes were compared with published correlations. Experimental results for the two-dimensional tube were predicted well by the correlations, but no suitable correlation was found for the three-dimensional tube so one was developed.

Three Dimensional Velocity Measurements in an Automotive-Size Evaporator Using Particle Image Velocimetry

The flow distribution inside of an evaporator is important to fully understand in order to optimize the design of the evaporator. A stereoscopic particle image velocimetry (PIV) system was used to measure single-phase water flow in a Plexiglas model of an automotive-sized evaporator. The evaporator is a “U-shape” type. Flow enters the inlet header and travels through a series of 26 parallel rectangular tubes. The tubes have a width of 15.5-mm, a flow gap (thickness) of 0.9-mm, and a length of 231-mm. The flow then enters the upper header and flows through another series of 26 parallel tubes to the outlet header. PIV measurements were only made within the headers due to the small size of the tubes, however detailed results were observed. In addition to the single-phase experimental results, computational fluid dynamics (CFD) simulations were conducted using the commercially available software Fluent, and the results compare well to the experimental results. Further work was conducted by injecting nitrogen into the flow to obtain two-phase flow under adiabatic conditions. Due to high vapor volume fractions, PIV could not be used for flow measurement, but a volume collection method was used to measure the flow of water through each tube. Significantly different flow distributions were observed at different inlet volume fractions of nitrogen and further investigation is underway.Copyright

Evaluation of Heat Transfer and Pressure Drop for the Heater-Core in an Automotive Heat Pump System

The heat transfer and pressure drop results for a heater-core of an automotive system are presented and discussed in this article. The heater-core is a type of compact heat exchanger that is used as part of an automobile heating-cooling system for heating the passenger cabin on cold seasons. The automotive heating-cooling system in this study includes a standard refrigeration cycle consists of a condenser, an evaporator, a compressor and an expansion valve using the refrigerant R134a as the working fluid. Furthermore, the system uses two separate secondary fluid loops using a 50% glycol-water mixture to exchange energy with the main refrigeration loop. During the cold weather season, the system is operated in the heat pump mode and one of the fluid loops is used to transfer heat from the condenser to the heater-core for heating the passenger cabin. The heat transfer from the heater-core to the passenger cabin is accomplished using air flow through the heater-core openings in an unmixed and cross-flow fashion. The air-side of the heater-core has a unique louver system that is intended to enhance the air-side heat transfer while the glycol-side has a twisted wire inserts to enhance flow turbulence and heat transfer. Semi-empirical correlations for the heat transfer and pressure drop for both glycol-water mixture and air flows in the heater-core are proposed. The flow of the glycol-water mixture in the heater-core is a single-phase flow within a bundle of parallel circular tubes with the twisted wire inserts. The flow of air through the heater-core is approximated as a flow across a finned-tube compact heat exchanger with continuous plate-fins. A modified Wilson plot technique is applied to determine correlations for heat transfer on both glycol-water mixture and air sides. The frictional pressure drop on the glycol-side is calculated from the total measured pressure drop and adjusted for pressure drops within manifolds and inlet/outlet ports. The results for the heat transfer and pressure drop analyses are finally plotted, discussed and compared with the relevant previous studies. These results show that the heat transfer rate is increased in the glycol-side due to the twisted wire inserts, in comparison with the smooth circular tubes. The air-side heat transfer rate is also enhanced due to the louvers in the air passages, as compared to flat-plate fins in compact heat exchangers.Copyright