Vacuum Pressure Levels Research Articles

A study of the thermal conductivity of granular silica materials for VIPs at different levels of gaseous pressure and external loads

Fast and reliable methods for the determination of thermal properties of core materials for vacuum insulation panels (VIPs) are needed. It is of great importance to know the thermal performance of a VIP core at different levels of vacuum and external loads. In this study a new self-designed device, consisting of two cylindrical cavities connected to a Transient Plane Source instrument, is used to determine the thermal conductivity of low-density nanoporous silica powders, from atmospheric pressure down to 0.1mbar while applying different levels of external pressure up to 4bars. The study includes a brief theoretical discussion of methods. The TPS is validated through comparison with available data for commercial silica as well as through independent stationary measurements with a hot plate apparatus and with a Transient Hot Bridge method. The different materials illustrate clear but different trends for the thermal conductivity as a function of the level of vacuum and external pressure. The analysis of experimental results shows that the transient methods are less suitable for measuring the thermal conductivity of low-density silica powders, especially for the cases when the density is less than a limit at which the heat transfer by radiation becomes dominant compared to pure conduction.

Properties of single crystal piezoelectric Ca3TaGa3Si2O14 and YCa4O(BO3)3 resonators at high-temperature and vacuum conditions

Piezoelectric single crystals Ca3TaGa3Si2O14 (CTGS) and YCa4O(BO3)3 (YCOB)were grown by the Czochralski pulling method and specially cut into three types of resonator samples with different vibration modes (CTGS lateral mode, YCOB thickness shear mode and planar mode). The dielectric, elastic and piezoelectric properties of the resonators have been investigated as functions of temperature (up to 800°C) and vacuum pressure (down to 10−10Bar). It was found that all the crystal resonators have excellent stability in the test range of vacuum pressure level; and their fundamental resonant frequencies exhibit temperature dependent characteristics, some of which were linear. It is also found that for CTGS lateral mode, YCOB thickness shear mode and YCOB planar mode resonators, the relative dielectric permittivity ε11T/ε0, ε22T/ε0, ɛT/ɛ0 are 17.0–18.2 (17.6±0.6), 13.5–24.4 (19.0±5.5), 14.0–16.9 (15.4±1.5); the elastic coefficients S11E, S66E, SE are 8.8–9.3 (9.1±0.3), 16.3–17.3 (16.8±0.5), 8.3–9.2 (8.8±0.5) (pm2/N); and the piezoelectric coefficients d11, d26, d are 3.7–3.9 (3.8±0.1), 8.8–10.9 (9.9±1.0), 3.3–3.9 (3.6±0.3)(pC/N), respectively. Mechanical quality factor Q was found to be 4500–7000 and 6000–11,000 for CTGS and YCOB, respectively. The excellent property stability of these CTGS and YCOB piezoelectric resonators, especially the thickness shear mode, make them very promising candidates for sensing applications at high-temperature and high-vacuum conditions.

Numerical simulation of tube-tooling cable-coating with polymer melts

This study investigates the numerical solution of viscous and viscoelastic flows for tube-tooling die-extrusion coating using a hybrid nite element/nite volume discretisation (fe/fv). Such a complex polymer melt extrusion-draw-coating flow displays a dynamic contact line, slip, die-swell and two separate free-surfaces, presenting an inner and outer conduit surface to the melt-coating. The practical interest lies in determining efficient windows for process control over variation in material properties, stressing levels generated and vacuum pressure levels imposed. The impact of shear-thinning is also considered. Extensive reference is made throughout to viscous inelastic counterpart solutions. Attention is paid to the influence and variation in relevant parameters of Weissenberg number (We), solvent-fraction (β) and second normal difference (N 2) (ξ parameter for EPTT). The impact of model choice and parameters upon field response is described in situ through, pressure-drops, rates of deformation and stress. Various numerical alternative strategies, their stability and convergence issues are also addressed. The numerical scheme solves the momentum-continuity-surface equations by a semi-implicit time-stepping Taylor-Galerkin/pressure-correction (TGPC) finite element (parent-cell) method, whilst invoking a sub-cell cell-vertex fluctuation distribution finite volume scheme for the constitutive stress equation. The hyperbolic aspects of the constitutive equation are addressed discretely through upwind Fluctuation Distribution techniques, whilst temporal and source terms are consistently accommodated through medium-dual-cell schemes. The dynamic solution of the moving boundary problem may be resolved by either separating the solution process for each free-surface section (decoupling), or coupling both sections and solving simultaneously. Each involves a surface height location method, with dependency on surface nodal velocities and surface element sections; two such schemes are investigated. Dedicated and localised shock-capturing techniques are introduced to handle solution singularities as disclosed by die-swell, slip and moving contact lines.

Analysis and Assessment of Measurements Results of Electric Power Demand in a Model Vacuum-assisted Moulding Installation

Abstract In this article the main problems related with the proper choice of the design and operation parameters of vacuum installation in vacuum moulding system have been discussed. In such system a vacuum are generated using electric-driven vacuum pumps. The aim of the experiment is to evaluate the performance of a vacuum system basing on registered plots of selected electric power parameters of the power-supplying system of the pumps with parallel measurements instantaneous values of pressure in selected points of model stand. The measurements system for power-supply unit has incorporated the recorders of instantaneous current and voltage values. Following the suitable numerical procedure, the experimental data are analysed to yield mathematical relationships between the variations of the generated vacuum pressure levels and variations of selected electric power parameters. According to the authors, the applied measurements system of the parameters of a vacuum-assisted installation may become an effective and easy practical method of evaluating the performance of such installations, used also in industry.

Applications of Monitoring of Electric Power Parameters to Performance Evaluation of a Model Vacuum-Assisted Moulding Installation

Applications of Monitoring of Electric Power Parameters to Performance Evaluation of a Model Vacuum-Assisted Moulding Installation Vacuum conditions in foundry installations are generated using electric-driven vacuum pumps. The purpose of the experiment is to evaluate the performance of a vacuum-assisted system for compaction of moulding sand basing on registered plots of selected electric power parameters of the power-supplying system of the pumps. Model testing done on an experimental vacuum system installation, power-supplied from a system incorporating the recorders of instantaneous current and voltage values. Following the numerical procedure, the experimental data are analysed to yield mathematical relationships between the variations of the generated vacuum pressure levels and variations of selected electric power parameters. Registered and computed values of selected parameters: instantaneous and RMS voltage and current values, active, reactive and apparent power levels and power coefficient allow for diagnosing the adequacy and reliability of the system operation. According to the authors, the applied monitoring of the power parameters of a vacuum-assisted installation may become an effective and easy practical method of evaluating the performance of such installations, used also in foundry plants.

MATHEMATICAL MODELS FOR VACUUM‐MICROWAVE CONCENTRATION BEHAVIOR OF PINEAPPLE JUICE

ABSTRACTLaboratory scale microwave vacuum was designed and developed to concentrate pineapple juice. Pineapple juice was concentrated from initial concentration of 12.26 ± 0.11 to 61.21 ± 1.47°Brix at three different microwave power densities (0.451, 0.925 and 1.536 W/g) under four levels of vacuum pressure (200, 300, 400 and 500 mbar). The effect of operating conditions (vacuum pressure and microwave power density) on evaporation rate of pineapple concentrate was investigated to select the most appropriate empirical relationships for the prediction of change in pineapple juice concentration during microwave‐vacuum evaporation (MVE) under different conditions. The experimental data were fitted to various empirical models (Lewis, Page, modified Page, Henderson and Pabis, Two terms, Two term exponential, Logarithmic and Wang and Singh) to determine the most appropriate representation of the concentration behavior of pineapple juice under vacuum‐microwave environment. The empirical models were compared according to three statistical parameters, the correlation coefficient (R2), reduce chi‐square (χ2) and root mean square error (RMSE). The modified Page model gave the highest R2 and lowestχ2 and RMSE values and thus, predicted evaporation characteristics of the pineapple concentrate more accurately. Based on the multiple regression analysis, the relationship of model constants and coefficient was expressed in terms of microwave power density and vacuum pressure. Validation of the selected model was determined with the experimental data. The result showed the suitability of the modified Page model in describing evaporation rate of pineapple concentrate.PRACTICAL APPLICATIONSEvaporation is a type of heat transfer, which involves the evaporation of a volatile solvent such as water from a nonvolatile solute material in a solution. Microwave vacuum applied in evaporation process has the advantage of highly efficient technology because it can improve energy efficiency and product quality. This study investigated the effect of microwave power density and vacuum pressure on the concentration of pineapple juice in order to select the suitable mathematical model for the representation of concentration change.

Vacuum-packaged piezoelectric vibration energy harvesters: damping contributions and autonomy for a wireless sensor system

This paper describes the characterization of thin-film MEMS vibration energy harvesters based on aluminum nitride as piezoelectric material. A record output power of 85 µW is measured. The parasitic-damping and the energy-harvesting performances of unpackaged and packaged devices are investigated. Vacuum and atmospheric pressure levels are considered for the packaged devices. When dealing with packaged devices, it is found that vacuum packaging is essential for maximizing the output power. Therefore, a wafer-scale vacuum package process is developed. The energy harvesters are used to power a small prototype (1 cm3 volume) of a wireless autonomous sensor system. The average power consumption of the whole system is less than 10 µW, and it is continuously provided by the vibration energy harvester.

Estimation of Moisture Ratio of a Mushroom Undergoing Microwave-vacuum Drying Using Artificial Neural Network and Regression Models

The drying rate of a mushroom undergoing microwave-vacuum (MV) drying (MVD) was controlled by moisture dissipation and was dependent on vacuum pressure levels. The main objective of this work was to develop artificial neural network (ANN) model to predict moisture ratio of MV-dried mushrooms. One-hidden-layer feed-forward ANN models were trained and validated with experimental data. The Levenberg-Marquardt algorithm was utilized in regulating the ANN model weights and biases. Inputs for ANN models were vacuum pressure and drying time. Output from ANN models was moisture ratio at a given drying time. Reduced chi-square (X 2) and root mean square error (RMSE), and residual sum of squares (RSS) of the results from ANN models were calculated and compared with those of a modified Page's model (an experimental-based mathematical model), which is commonly used in the literature. The X 2, RMSE, and RSS of the ANN model (2.272 x 10 -5, 4.023 x 10 -3, and 3.204 x 10 -3, respectively) were found to be lower than those of the modified Page's model (6.692 x 10 -4, 2.561 x 10 -2, and 12.98 x 10 -2, respectively). These results indicate that the feed-forward ANN model represented the drying characteristics of mushrooms better than the modified Page's model. Therefore, the ANN model could be considered as a better tool for estimation of the moisture content of mushrooms than by the modified Page's model.

Parallel Dynamic Artificial Neural Network for Temperature and Moisture Content Predictions in Microwave-Vacuum Dried Tomato Slices

Temperature (T) and moisture content (MC) of non-homogenous food undergoing microwave-vacuum (MV) drying (MVD) are directly dependent on microwave power, vacuum pressure, and the product's physical properties. A two-hidden-layer Artificial Neural Network (ANN) model was developed in an earlier study to predict temperature and moisture content of the product at a given time based on the present state of product conditions and process control parameters. This approach either provided lowest error in temperature prediction or in moisture content prediction but not the lowest error in both the prediction parameters simultaneously. The main objective of this work was to improve the performance of the ANN model for temperature and moisture content predictions in MV dried samples. Experimental data obtained from MVD of tomato slices at different drying conditions was normalized and divided into two groups for training and validating. The parallel dynamic ANN model consisted of two double-hidden-layer feed-forward ANN models with varying node numbers (10, 20, and 30). These models were separately trained, simultaneously for moisture content as well as temperature, with the Levenberg-Marquardt algorithm. Inputs for the ANN models were magnetron on-off status, vacuum pressure, temperature, and moisture content at time `ti'. The previous temperature and moisture content data at time `ti-1, i-2, , i-n' where n = 0, 10, 20, and 30 were also added to the input layer. Outputs from the ANN models were temperature and moisture content at time `ti+1'. The results indicated that the dynamic ANN model working in parallel with the previous temperature and moisture content data provided results that are more accurate and required less training time than those of ordinary ANN models. Model simulation may supply essential information regarding temperature and moisture content of non-homogenous foods corresponding to microwave power and vacuum pressure levels to the predictive control system. Therefore, improved drying efficiencies and thermal damage prevention may be achieved.

Preparation of dry honey by microwave–vacuum drying

Microwave–vacuum (MWV) drying was investigated as a potential method for obtaining high-quality dried honey. Liquid honey was heated and dehydrated in a MWV dryer to a moisture content less than 2.5% within about 10 min. The drying curves and the temperature changes of samples were tested during MWV drying at a different of microwave power, vacuum pressure levels and sample thicknesses. Fructose, glucose, maltose and sucrose contents in the liquid and dry honey were determined by high-performance liquid chromatography (HPLC). The volatiles in the liquid and dry honey were concentrated by solid-phase microextraction (SPME), separated and identified by gas chromatography–mass spectrometry (GC–MS). A sample thickness of less than 8 mm and a vacuum pressure of 30 mbar were identified as the better parameters for the MWV drying. The core temperatures of the sample were about the same as the surface temperatures, the temperature changes were from 30 to 50 °C with higher dehydration rates while no darkening of the honey took place during MWV drying. There were no significant changes on the contents of fructose, glucose, maltose and sucrose in the honey after MWV drying. The volatile acids, alcohols, aldehydes and esters made up the bulk of the identified aroma compounds of the used liquid honey and the content of alcohols and the esters changed slightly. The acids decreased markedly whereas the aldehydes and the ketones increased remarkably in the honey dehydrated by MWV drying.

Optimisation of the seed spacing uniformity performance of a vacuum-type precision seeder using response surface methodology

The objective of this study was to optimise the seed spacing uniformity performance of a precision seeder using response surface methodology (RSM) and to verify the optimum levels of the variables. The variables considered in the study consisted of the vacuum on the seed plate, the diameter of seed holes and the peripheral speed of the seed plate. Cotton seeds were used for sowing and experiments conducted in this study were based on the central composite design (CCD), one of the designs in RSM. Data obtained in the laboratory were divided into three different groups in order to obtain values of the multiple index, quality of feed index and miss index. An additional performance criterion was also proposed and used as an indicator of the sowing performance. This was the root-mean-square deviation from the theoretical seed spacing. The data obtained in the laboratory were then used to develop functions in polynomial form that allowed the calculation of the optimum level of each independent variable considered in the study. The optimum levels of vacuum pressure and the diameter of holes for precision seeding of cotton seeds were found to be around 5.5 kPa and 3 mm, respectively. No optimum value was obtained for the peripheral speed of the seed plate. It was found that the lower the peripheral speed of the plate, the higher is the performance.

Drying Kinetics and Quality Characteristics of Microwave-Vacuum Dried Saskatoon Berries

Saskatoon berry (Amelanchier alnifolia) is a high moisture content fruit crop, harvested at 75–80% moisture content (w. b.) and dried to lower moisture contents for a safe, extended shelf-life and for further usage in food applications. Saskatoon berries were driedfrom an initial moisture content of 75.5% to around25% (w.b.) using microwave-vacuum drying. In this study, saskatoon berries were dried at different microwave power levels P10 (745 W), P7 (514 W) andP5 (374 W) and vacuum pressure levels V20 (67.0 kPa) and V10 (33.5 kPa). The vacuum pressure levels did not greatly influence the drying time of the saskatoon berries compared to the microwave power levels. Increasing the microwave power level produced faster drying times for the samples. After 12 min of drying at power level P10, the moisture contents of the samples at vacuum pressures of 33.5 and 67.0 kPa were 29.74 and 31.24%, respectively. However, after dryingfor 12 min at vacuum pressure of 33.5 kPa, the moisture contents atpower levels P10, P7 andP5 were 29.74, 51.41 and 61.95%, respectively. Polynomial and exponential models were fitted to the drying data. The exponential model provided a better fit for the drying data at the lower vacuum pressure, with higher R2 and lower SE compared to the polynomial model. On the other hand, the polynomial model provided a better fit for the drying data at the higher vacuum pressure. The vacuum pressure and the microwave power levels did not significantly change the yellowness of the samples at indicated by the Ab values. However, microwave-vacuum drying produced significant color changes in the L (white = 100 to black = 0) and a (green = -a to red = +a) values as indicated by AL and Aa. Total color difference was dependent on microwave power and vacuum pressure levels. Water activity of the samples after drying ranged from 0.61 to 0.75 for moisture content range of 15.68 to 29.03%.

Microwave Vacuum Dryer Setup and Preliminary Drying Studies on Strawberries Carrots

A laboratory scale microwave vacuum dryer with the ability to record temporal variation of mass and temperature of a drying product was designed and built. The initial study was set up to investigate the effect of the position of a vacuum pressure control valve at two vacuum pressure levels, 6.5 and 13.3 kPa, with a fixed microwave power input of 1.5 W/g. Then, strawberry halves and carrot cubes (10 x 10 x 10 mm) were used for a preliminary study to investigate the effect on drying product temperature and the effect of input microwave powers (1, 1.5 and 2 W/g) at a fixed level of vacuum pressure (6.5 kPa). The position of the valve which allows air to pass through the vacuum container was found to provide shorter drying time and reduced the occurrence of water vapor condensation. The product temperature at the end stage of drying under continuous microwave mode was too high to ensure quality for both dried strawberry halves and carrot cubes.

Temperature Changes during Microwave-Vacuum Drying of Sliced Carrots

The temperature changes during microwave-vacuum drying of sliced carrots were investigated. Sliced samples were dried to 7–10% moisture content (wet basis) at a wide range of microwave power and vacuum pressure levels. The experiments showed that for sample thickness less than 8 mm, the core temperature of the sample was the same as its surface temperature, with uniform temperature distribution within the sample. However, for sample thickness more than 8 mm, temperature gradient developed along the thickness of the sample. The experiments also showed that, with the decrease of moisture content X w (dry basis), for samples with thickness ≤ 8 mm, the drying process of sliced carrots experienced three distinct periods: a warming-up period (X w = 7.68) without removal of moisture when the product temperature increased linearly with drying time until it reached the corresponding saturation temperature of water in the food at the vacuum pressure; a constant temperature period (2 ≤ X w < 7.68) in which most of moisture evaporated and flowed out of the sample efficiently with little resistance; and a heating-up period (X w < 2) in which the drying rate decreased and sample temperature increased rapidly. The mathematical models for predicting sliced sample temperature were also developed based on the energy conservation and regression of the experimental date.

OPTIMIZATION OF VACUUM PULSE OSMOTIC DEHYDRATION OF CANTALOUPE USING RESPONSE SURFACE METHODOLOGY

The optimum levels of vacuum pressure, concentration of osmotic solution and dehydration time for vacuum pulse osmotic dehydration of cantaloupe were determined by response surface methodology (RSM). The response surface equations ( P < 0.05 and lack of fit > 0.1) explain the 97.6, 88.0 and 97.1% of the variability in weight loss, water loss and °Brix increase, respectively, at 95% confidence level. The canonical analysis for each response indicated that the stationary point is a saddle point for weight loss and °Brix increase, and a point of maximum response for water loss. The region that best satisfied all the constraints (low values in weight loss and °Brix increase, and high value in water loss) is located within the intervals from 49.5 °Brix to 52.5 °Brix for concentration and from 75 min to 84 min for dehydration time at a vacuum pulse of 740 mbar.

Microwave–vacuum drying kinetics of carrot slices

The kinetics of microwave–vacuum drying of thin layer carrot slices was studied by introducing a theoretical model. The model is based on the energy conservation of sensible heat, latent heat and source heat of microwave power. The model was tested with data produced in a lab microwave–vacuum dryer in which the materials to be dried could rotate in the cavity. The theoretical and experimental drying curves showed that the theoretical model was in agreement with experimental data, and drying rate was a constant until the dry-basis moisture content X s was about 2. As 1⩽ X s <2, the experimental drying curves showed a little deviation from the theoretical drying curves. While X s <1, the experimental drying curves showed a sharp deviation from the theoretical drying curves. To predict the changing of moisture content with time by the theoretical model in the period of X s <2, a correction factor, ϕ, was introduced and obtained using non-linear regression analysis. The investigation involved a wide range of microwave power and vacuum pressure levels. Both the theoretical model and experimental data also showed that the drying rate was linear to the microwave power output, and inversely proportional to the first order of latent heat of evaporation for water at the vacuum pressure of P.

Experimental Study on Microwave Vacuum Drying of Two-Porosity Level Media

Microwave vacuum drying experiments were performed on a laboratory scale dryer with a two-level porosity material: a packed bed of porous alumina beads. The incident microwave power and the vacuum pressure level were fixed, the main varying parameters being the beads diameter and porosity, and the mean pore diameter. The drying kinetics and the evolution of the product temperature are presented. The drying kinetics can be divided into two main periods. The first one corresponds to the drying from the bed voids according to evaporation mechanism that we describe with a stagnant film law. The second one corresponds to the drying from the particle pores and we divide it into two parts: we suggest that the former is dominated by capillarity driven moisture transport, and the latter is limited by the desorption kinetics of the few water layers left.

MICROWAVE VACUUM DRYING OF POROUS MEDIA: VERIFICATION OF A SEMI-EMPIRICAL FORMULATION OF THE TOTAL ABSORBED POWER

Vacuum drying experiments were performed on a laboratory scale dryer with two porous materials: a packing of initially saturated glass beads and of unsaturated pharmaceutical granules. Several incident powers and two vacuum pressure levels were tested in order to demonstrate different drying mechanisms. The drying kinetics, temperature of the product and the absorbed power are presented and the coupling between the drying rate and the absorbed power is shown. A semi-empirical formulation of the total absorbed power is proposed taking into account the water content, the temperature and the dielectric properties of each phase. This formulation is based on a quasi-static assumption which allows the local electric field inside the material to be expressed with an analytical equation.

MICROWAVE FREEZE DRYING CHARACTERISTICS OF BEEF

ABSTRACT Microwave freeze drying characteristics of unsaturated raw beef were studied experimentally for various levels of electric field strength, vacuum pressure, sample thickness and initial saturation. The results show that drying time is proportional to initial saturation and inversely proportional to the electric field strength and sample thickness. The effect of vacuum pressure on drying time is negligible. Some advantages of microwave freeze-drying over conventional freeze drying with radiant heating are pointed out

THE EFFECT OF PROCESS CONDITIONS ON THE DRYING KINETICS AND REHYDRATION CHARACTERISTICS OF SOME MW-VACUUM DEHYDRATED FRUITS

ABSTRACT The effect of drying conditions, namely, microwave power, vacuum pressure, as well as sample geometry, on the drying kinetics and rehydration characteristics of five microwave vacuum dehydrated fruits (apple, avocado, mushroom, pear and strawberry) was studied. The investigation involved a wide range of microwave power, vacuum pressure and sample size levels. The drying rate and the rehydration ratio, a measure of rehydration characteristics, were found to depend on the drying conditions. An empirical mass transfer model, involving a characteristic parameter for each process (drying and rehydration( as a function of process variables, namely, microwave power, vacuum pressure and sample size, was also tested with the data obtained in a microwave oven equipped with vacuum apparatus. Furthermore, a comparison of rehydration properties of microwave vacuum and conventionally dehydrated products, was conducted.