Air Flow Humidity Research Articles

High sensitivity CMOS humidity sensors with on-chip absolute capacitance measurement system

In this paper we report a new approach for an integrated humidity sensor fabricated using an industrial CMOS process with a measurement technique for evaluating on-chip floating capacitances. The sensor is based on a capacitive element covered with polyimide as humidity sensitive material. The circuit produces an output current which is precisely related to the absolute value of the capacitance and is insensitive to parasitic capacitances to ground, offsets and charge injection from switches, which enhances the sensitivity of the device. The response to relative humidity (r.h.) is highly linear and a maximum sensitivity of 0.9% (%r.h.) −1 is achieved, i.e. six times better than our previous CMOS humidity sensors ( Sensors and Actuators A, 25–27 (1991) 509–512). The hysteresis is reduced to less than 1% r.h. and the temperature coefficient to less than 0.1% r.h. °C −1. Cross-sensitivity to flow of humid air is below 2% r.h. for flow rates up to 15 m/s. Our measurement technique can be used for evaluating on-chip capacitive microsensors, whenever exact knowledge of the device capacitances is crucial.

Evaporation during preparation of unsupported thin vitrified aqueous layers for cryo‐electron microscopy

Evaporation of water cannot be fully avoided when an unsupported thin vitrified film of an aqueous suspension is prepared for cryo-electron microscopy. This results in increasing concentration of solute which could affect the observed material. We have quantitatively studied this effect by measuring the contrast of polystyrene spheres in a metrizamide solution. The drying effect is generally negligible when specimens are prepared on a hydrophilic perforated support but it is frequently important when hydrophobic films are used instead. A flow of humid air, double blotting with minimal exposure of the thin liquid film to the atmosphere, or an automatic plunger optimizing the blotting conditions are simple methods for reducing drying effects. With this third device acting on a hydrophilic supporting film, the increase of solute concentration is limited to less than 20%.

Experimental investigation of a cooled thick fin in dry and humid air flows

The results of a study are reported in which a cooled, thick vertical fin was tested in a closed loop tunnel with and without condensation from the air flowing over it. In particular, the temperature distributions for the dry and wet fin cases, together with the condensate film thickness in the wet fin case, were investigated. From a flow visualization investigation, it was found that the boundary layer separates at the leading edge, resulting in a higher air heat transfer coefficient. The wet fin test results also indicated that the mode of condensation was dependent on fin surface characteristics and that the wet fin performance was governed by the air flow parameter. Within the laminar air flow range, the condensate film flowed downward under the action of gravity. However, at higher air velocities, both gravity and shear forces affected the condensate flow, a variation in the condensate film in the direction of air flow being noticed.

Fin performance with condensation from humid air: a numerical investigation

The performance of cooling coils in air conditioning equipment is affected by condensation which may form on the fins of the coil tubing. Extending well established theories, a numerical method has been evolved for condensation from a humid air flow. Considering a cooled vertical fin, a heat and mass transfer analogy is applied to the conjugation of convected heat and mass in a buffer layer, adjacent to the condensate layer, with heat conduction through the fin. The fin temperature distribution, the condensate film thickness and the fin effectiveness are predicted, numerically, for a fin in a laminar humid air cross flow. The variations in these values show that condensation has a substantial influence on fin performance, the reduction in fin effectiveness being a function of the additional heat associated with latent heat when condensation occurs.

Condensation and isothermal water transfer in cement mortar: Part II - transient condensation of water vapor

The transient wetting of a mortar sample swept by a flow of humid air is experimentally studied at temperatures of 30 and 55°C. The water content profile shape and evolution are found to be very different from those which were observed during imbibition. The boundary condition on the exposed wall of the sample is examined. A convenient evolution of the coefficient of diffusion with water content is explored. This coefficient is interpreted in terms of pure vapor diffusion, even at relatively high water contents. But its values at low water content and its temperature dependence are inconsistent. Additional explanations are then considered with the assumption that the vapor condensation in the medium is not an equilibrium process between vapor and liquid phases. The physical origin of such a nonequilibrium process is discussed. A tentative set of transfer and phase change coefficients is proposed in order to describe the experimental data by means of numerical simulation. Then, some aspects of the imbibition processes are re-examined, taking into account the consequences of a nonequilibrium condensation.

Frost formation and defrosting on tube-array evaporators in a fluidized bed and an impinging jet

Frost formation on evaporators and the resultant choking of the evaporators are perplexing problems in heat pump systems using ambient air as a heat source. In the present study, heat and mass transfer, frost formation, and defrosting of single-row cooled tubes were investigated experimentally in a uniform humid air flow, an impinging air jet, and a fluidized bed with a multislit distributor and static bed height of 10 mm, respectively. Because of frost formation, the heat transfer performance for the impinging jet decreased with time. With Teflon-coated tubes in the fluidized bed using glass beads 0.92 mm in diameter, it was found that frost-free running is possible under comparatively wide conditions of heat flux and superficial velocity, and that the heat transfer performance is about five times as large as that for uniform airflow.

Second Law Analysis of Solid Desiccant Rotary Dehumidifiers

This paper presents a second law analysis of solid desiccant rotary dehumidifiers. The equations for entropy generation for adiabatic flow of humid air over a solid desiccant are developed. The generation of entropy during operation of a rotary dehumidifier with infinite transfer coefficients is investigated and the various sources of irreversibility are identified and quantified. As they pass through the dehumidifier, both the process and regeneration air streams acquire nonuniform outlet states, and mixing both of these air streams to deliver homogeneous outlet streams is irreversible. Transfer of mass and energy between the regeneration air stream and the desiccant matrix occurs across finite differences in vapor pressure and temperature and these transfer processes generate entropy. The second law efficiency of the dehumidifier is given as a function of operating conditions and the effect of finite transfer coefficients for an actual dehumidifier is discussed. It is shown that operating the rotary dehumidifier at conditions that minimize regeneration energy also yields a local maximum for the second law efficiency.

Frost formation and defrosting of tube-array evaporators in a fluidized bed and an impinging jet.

Frost formation on and choking of evaporators are perplexing problems in heat-pump systems using ambient air as the heat source. In the present study, heat-and mass-transfer, frost formation and defrosting of single row cooled-tubes were investigated experimentally in a uniform humid air flow, an impinging air jet, and a fluidized bed with a multi-slit distributor and a static-bed height of 10 mm, respectively. The heat transfer performance for an impinging jet decreased with time, because of the frost formation. As for teflon coated tubes in the present fluidized bed using glass beads of 0.92 mm in diam., it was found that non-frost operation is possible under comparatively wide conditions of heat flux and superficial velocity, and that the heat transfer performance is about five times that for the uniform air flow.

A further study of the effects of water vapor concentration on the rate of combustion of an artificial graphite in humid air flow

Test specimes of graphite (disk-shaped, 10 mm in diameter and 1 mm in thickness) were burned in the stagnation region of an impinging oxidizer flow under atmospheric pressure; the velocity gradient of the impinging oxidizer flow was 100 s −1. The surface temperature of the test specimen was kept constant during each experimental run by an external heat source in a range of 1200–1700K. Three groups of oxidizers containing water vapor were used. The first group consists of humid air having six H 2O concentrations (0.0681, 0.641, 3.13, 3.63, 5.15, 7.41 mol m −3 at 0.101 MPa and 320–339K); the second group, O 2-recovered humid air having a fixed O 2 concentration and five H 2O concentrations equal to, respectively, the higher five of those for the aforementioned humid air; the third group, N 2-enriched air having a fixed H 2O concentration and five O 2 concentrations equal to, respectively, the higher five O 2 concentrations for the aforementioned humid air. The following results have been obtained. (1) When the water vapor concentration in the oxidizer flow is low, water vapor actively suppresses the combustion rate. (2) As the water vapor concentration exceeds a certain value, the effect of water vapor to suppress actively the combustion rate diminishes. (3) When the water vapor concentration is high, water vapor actively augments the combustion rate in the high surface temperature range.

高性能エアフィルタのか酷時健全性試験 Ｉ ＤＯＰエアロゾルの捕集性能

Collection efficiencies of high efficiency particulate air (KEPA) filters for dioctyl phthalate (DOP) aerosol were measured by using a laser particle spectrometer and a diffusion battery (DB)/ condensation nucleus counter (CNC) submicron sizing system to obtain the basic information to prove the performance of HEPA filters under severe conditions. Test parameters were such as DOP aerosol diameter, air flow rate, temperature and humidity of air flow and the number of filtering stages.It was found that the most penetrating particle size through HEPA filters ranged 0.10-0.15μm in diameter, and the collection efficiency at 0.3μm which is generally applied for the efficiency of HEPA filters was about an order of magnitude higher than that for the most penetrating particles. Among the parameters governing the conditions of air flow tests, the flow rate was most effective on the collection efficiency, whereas the influences of temperature and humidity were insignificant. The collection efficiencies of multi-stage HEPA filters were almost consistent between the first and the second filters for the same sized particles. Besides these, indispensable data for estimating collction efficiencies of HEPA filters such as, charge of DOP aerosol, the effect of pinhole in the filters etc. were obtained.

Prediction of Energy Requirements and Drying Times for Surface Drying Fresh Produce

FOR numerous fresh fruits and vegetables, drying of surface adhering water is required to facilitate materials handling and wax treatments. Using humidity ratio difference and air flow rates as manipulated variables, a computer program and a graphical approach were developed to predict required drying time. Modeling results were extended to investigate air recycling and the relationship of recycling on energy requirements.

Transient analysis of the thermal and moisture physical behaviour of building constructions

For the transient analysis of the thermal and moisture conditions in multilayer constructions a numerical algorithm and a computer program based on the Crank-Nicholson method and quasi-linearization are are formulated. Constitutive equations for simultaneous heat and mass transfer in porous material are derived from the integrated mass, momentum and energy balance equations using the volume averaging technique. The temperature and moisture content are used as transport potentials due to general practice. In energy balance equations the conduction, convection and accumulation of heat and heat sources due to interstitial phase changes are considered. In moisture balance equations the accumulation of moisture, the diffusion flow of water vapour, the capillary and surface diffusion flow of liquid water and the viscous flow of humid air and water are considered. The boundary layer and interfacial balance equations are derived, too. The accuracy of the numerical algorithm is compared with an analytical solution for thermally semi-infinite body. The validity of the simulation method is verified by two experiments.

The effects of water vapor concentration on the rate of combustion of an artificial graphite in humid air flow

Test specimens of graphite were burned in a stagnation region of an impinging oxidizer flow under atmospheric pressure. The shape of the test specimen was a disk of 10 mm in diameter and 1 mm in thickness. The mole fractions of water vapor in air were 0.0022, 0.017, and 0.08; the velocity gradients 22.0, 88.8, and 343 s −1. The surface temperature of the test specimen was kept constant during each experimental run by an external heat source in a range of 1200–1600K. The following results have been obtained: (1) Even when the water vapor concentration in humid air is high, the burned mass increases linearly with the burning time up to 120 s. (2) The increase in the water vapor concentration in humid air reduces the combustion rate in a surface temperature range of 1300–1600K. (3) For the decrease in the oxygen concentration caused by the increase in the water vapor concentration up to a certain value, the combustion rate diminishes to a greater extent than would be found for simple dilution of oxygen.

An experiment on transient heat and mass transfer in humid air flow

This paper reports the results of a preliminary investigation of transient condensation and evaporation for humid air flow. Using the principle of Newtonian heating, a novel experimental procedure has been developed for the determination of the instantaneous heat transfer coefficients and mass transfer rates and their dependence on time. Tests were made on a pre-cooled cylinder of negligible thermal resistance in a cross flow of humid air over a Reynolds number range 9300–22200. Provisional comparison with previous data for condensation in the steady-state case has been made and indicates that the proposed method may prove a valuable alternative to the steady-state technique of measurement in this area of study.

EFFECT OF SMOKEHOUSE TEMPERATURE, HUMIDITY AND AIR VELOCITY ON RATE OF HEATING AND QUALITY OF FRANKFURTERS

The effects of air velocity, relative humidity, temperature and presence of glucono delta lactone (GDL) on the quality of frankfurters, were evaluated. Rate of heating was inversely proportional to rate of weight loss during processing. High humidity processes at slow air flows had the fastest heating rates; however, quality was unacceptable because of fat separation and pale color. A process where the smokehouse temperature was slowly increased by 6°C increments, and where a low relative humidity and high air flow were used, gave the best quality. Color and texture was more desirable on products having higher shrink. The presence of GDL had no effect on the final color of products processed under the same conditions; however, maximum color appeared to develop earlier in samples containing GDL.

Thermal Tolerance of Beet Armyworm Moths, Spodoptera exigua: Effects of Age, Temperature Acclimation, and Gamma Radiation

Upper temperature limits were established for beet armyworm moths, Spodoptera exigua (Hubner) (Noctuidae), under controlled humidity–air flow conditions. Preliminary tests determined that a one-hour exposure to 45°C resulted in an approximately 50% reduction in the number of moths capable of flight following a 24-h recovery period. Subsequent tests using 45°C as the stress temperature indicated a rapid decline in thermal resistance of moths with increasing age. Moths maintained at each of 3 temperature regimes (22, 27, or 32°C) for 72 h post-emergence were inconsistent in their flight capabilities following exposure to the stress temperature. Developmentally acclimated moths whose pre-adult stages were reared at these 3 temperatures showed decreased heat resistance at the lower temperature, but inconsistent responses at the higher temperature. Flight capability of irradiated moths in the absence of heat stress was unaffected; however, flight capability following exposure to 45°C significantly decreased as dosage (20, 40, 80 krads) increased

Nonadiabatic Compressible Flow Of Humid AirAround Airfoils

The heat addition by the phase transition of water vapor in humid air, which changes the properties of compressible flows around airfoils, is investigated by numerical simulations. The computations are based on a recent small-disturbance approach by Rusak and Lee [11]. This approach describes the heat release by the nonequilibrium and homogeneous condensation process using the classical nucleation and droplet growth models. The nonlinear interaction between the heat transfer by the condensation, the kinetic energy of the compressible flow, and the thermal properties of the flow is explored and the similarity parameters which govern the problem are presented. The results of the calculations show good agreement with available simulations using the Euler equations. The similarity rules are used to suggest various flow cases which have a sufficiently close behavior of both the flow dynamics and the condensation properties. It is shown that nonadiabatic compressible flows of humid air may be applied to augment the aerodynamic performances of airfoils used in fluid machinery.

Spontaneous Condensation in a supersonic Flow of Humid Air

The problem of the spontaneous condensation of water vapor in a supersonic flow of moist air is approximately solved in a way similar to those given previously 1) , on the basis of Becker-Doring's equation for the stationary rate of self-nucleation and also based on the law of droplet growth used before. Comparison of the theoretical prediction with experimental results given by Head 2) shows a considerably good agreement and thus it is revealed that major part of the delay of the appearance of condensation, from which some doubt has been cast over 3) the validity of Becker's formula in such cases, can be explained plausibly without the notion of time lag in the self-nucleation.