Bare Probe Research Articles

Heat Transfer from a dc Laminar Plasma-Jet Flow to DifferentSolid Surfaces

The heat flux distributions were measured by using transient method foran argon dc laminar plasma-jet flow impinging normally on a platesurface embedded with copper probes. Different powders were coated onthe probe surfaces and the effect of powder coatings on the heattransfer from jet flow to the probe surface was examined. Experimentalresults show that the maximum values of the heat flux to the probeincrease with the coating of fine metal powders, while for the surfacescoated with fine ceramic powders, the maximum values of heat fluxdecrease, compared with that to the bare copper probe surface.

Accurate measurements of thermal conductivity and thermal diffusivity of molten carbonates

Thermal conductivity and thermal diffusivity of molten carbonates (Li 2 CO 3 /K 2 CO 3 , Li 2 CO 3 /Na 2 CO 3 , and Li 2 CO 3 /Na 2 CO 3 /K 2 CO 3 ) were measured by the transient short-hot-wire method in the temperature range from 530 to 670 °C. Two types of probes were examined. One was a platinum short-hot-wire probe coated with a thin film of alumina (Al 2 O 3 ) to prevent current leakage and corrosion. The other was a bare gold short-hot-wire probe. In the case of the platinum probe, the quality of the coating deteriorates gradually during the measurements on molten carbonates because of their high corrosiveness. The deterioration of the quality of coating leads to relatively large errors in the measured thermal conductivity and thermal diffusivity, of around 9% and 50%, respectively. In the case of the bare gold probe, corrosion due to electrochemical reaction can be neglected, and the effect of current leakage into the molten carbonates can be estimated. This allows the temperature dependence of electric resistivity to be calibrated accurately and compared with existing data. By using such a gold probe without coating on the hot-wire surface, the thermal conductivity and thermal diffusivity of molten carbonates can be measured within errors of 3% and 12%, respectively. The gold probe can be used repeatedly without any reduction of quality.

Validation of current density measurements with a B-dot probe

The time varying magnetic flux density in an electrodeless high frequency discharge was measured with a bare wire B-dot probe immersed in plasma. Based on B-dot measurements, the spatial distribution of the current density was determined and integrated over the cross section of the discharge to determine the total discharge current. The total current determined from B-dot measurements was compared with total current measured externally with a current transformer. Agreement between the two methods was very good over a wide range of gas pressures and discharge currents. This work validates that a small, immersed B-dot probe does not significantly perturb the discharge current and can accurately determine current density in a low-pressure gas discharge.

Fiber optic evanescent field absorption sensor with high sensitivity and linear dynamic range

A novel fiber optic evanescent field absorption sensor having high sensitivity and linear dynamic range is proposed. High sensitivity is achieved by using a U-shaped probe while linear response is obtained by preventing surface reactivity of the silica core of the fiber with the solute molecules of the absorbing fluid. To prevent it, a thin film of a suitable polymer is developed on the bare core U-shaped probe. A good degree of reproducibility of the results is obtained.

Buoyancy-Induced Flow Through a Narrow Chamber Containing an Internal Heat Source: Comparison of Experimental Measurements and Numerical Simulations

Natural convection ventilation of a rectangular chamber containing a heated rectangular body has been investigated both experimentally and computationally. The heated square blockage within the partial enclosure was adjacent to the adiabatic lower surface of the chamber and the three exposed surfaces of the block were such as to give a constant heat flux. The air inlet and outlet were located at the bottom and top of the chamber vertical walls respectively, and were of equal area. Velocity measurements were performed using laser Doppler anemometry (LDA) with a one-component He–Ne laser connected to a burst spectrum analyser (BSA). Detailed velocity profiles were measured at the inlet, outlet and at several locations inside the chamber. Temperatures of the heated air at the outlet were measured with a chromel–alumel (type K) bare wire thermocouple probe. Three-dimensional laminar and turbulent (k–e model) numerical simulations were obtained by solving the governing equations using the computational fluid dynamics (CFD) code PHOENICS. Comparisons of experimental and computational results showed very good agreement in most of the flow field.

Electrostatic force microscope for probing surface charges in aqueous solutions.

A scanning force microscope was converted to an electrostatic force microscope by charging the usually neutral cantilever with phospholipids. The electrostatic force microscope was used to study surface electrostatic charges of samples in aqueous solutions. Lysozymes, DEAE-Sephadex beads, 3-propyltriethoxysilane-treated glass and mica were imaged in water or phosphate buffer with electrostatic force microscopy. The adhesion force measured when a charged probe and oppositely charged specimen interacted was up to 500 times greater than when a bare probe was used. This dramatic increase in measured adhesion force can be attributed to the energy required to break the salt bridges formed between the charged probe and the specimen. The use of phospholipids to functionalize the cantilever tip allows the incorporation of other biomolecules and ligands that can be used as biologically specific tips (e.g., receptors, drugs) for the study of intermolecular interactions.

Interstitial laser thermotherapy: Comparison between bare fibre and sapphire probe

A sapphire probe and a bare fibre were compared with respect to temperature control and distribution and light fluence in interstitial laser thermotherapy. Experiments were performed in processed liver using an Nd-YAG laser and output power levels of 1–4 W. The temperature was controlled at a distance of 10 mm using a feedback circuit with an automatic thermometry system and thermistor probes. With the sapphire probe, carbonization was rare at power levels of 1–2 W but was observed in half of the experiments at 3 W and in all experiments at 4 W. Using the bare fibre, carbonization was seen in almost all experiments. Absence of carbonization was associated with a moderate decrease in the penetration of light and excellent control of the temperature, whereas carbonization led to rapid impairment of light penetration and temperature control. In addition, the temperature gradient was smaller with the sapphire probe than with the bare fibre or when carbonization was absent. It is concluded that a diffuser tip, such as the sapphire probe, may be preferable to the bare fibre for interstitial laser thermotherapy because it gives a smaller temperature gradient and helps to avoid carbonization which results in preserved light penetration and improved temperature control.

Acoustic intensity techniques for airplane cabin applications

A technique to measure surface radiation from an airplane cabin in flight using a two-microphone acoustic intensity system is described. The technique addresses the problems of high background levels and surface absorption that have in the past complicated cabin radiation measurements. Two probe types were used: a bare probe for the reflective sidewall and ceiling regions and a shielded probe for the absorptive carpet. Laboratory tests were conducted to establish the accuracy and tolerance to background noise for the flight measurement system. From these tests the operating range for each probe was determined in terms of the difference in sound pressure and intensity levels. This difference, called the signal-to-noise indicator, was used to screen out flight data saturated by the background field. From the cabin surveys, several strong radiation areas, such as the ceiling panel and the air distribution and air return grills, were measured quantitatively. Small area sources were distinguished from adjacent areas, and other weak sources were identified.

An evaluation of limiter configurations in ZT-40M

Three limiter configurations were evaluated in the reversed-field pinch, ZT-40M. A bare graphite mushroom-shaped probe located in a pump port measured changes in the energy flux profiles resulting from three different limiter configurations. The first configuration was a graphite segmented poloidal ring limiter located 90° toroidally upstream from the mushroom probe, and it protruded into the plasma chamber ~ 0.95 cm beyond the bellows. The second configuration consisted of three graphite tiles which were positioned in the bottom of the convolution section immediately upstream from the mushroom probe. These tiles were electrically floating with respect to the metal liner and protruded radially inwards ~1.27 cm beyond the bellows. For the third configuration four graphite tiles were placed on each side of the pump port section where the mushroom probe was located. These tiles extended 25 cm in both toroidal directions from the pump port section and protruded ~ 0.95 cm into the plasma region. These tiles were also electrically floating. The introduction of the poloidal ring limiter resulted in the mushroom probe absorbing 28% less energy at the major radial position where the tip of the mushroom was even with the shadow of the liner bellows. The tile limiters produced an additional, but small, reduction in the energy absorbed at this location. The tiles produced an increase in the plasma resistance and radiation late in the discharge. Significant amounts of arcing occurred between the tiles and the liner. The experimental results led to a configuration which utilized four graphite poloidal ring limiters. These four limiters protected the pump port areas; however, the occasional increase in the liner temperature at a location approximately halfway between two limiters was not eliminated. The fraction of input energy radiated during the constant-current phase of the discharge increased from 20% to 30% with the four poloidal limiters, owing to an increase in electron density. During start-up, the energy radiated per electron was less for the four-ring case than for the case with no limiters, indicating that the discharge may have been cleaner with the four rings.

Detection of Breaking Events in a Wind-Generated Wave Field

Abstract Measurements were made of the surface elevation of a fetch-limited wave field (fetch 7 km, wind speed about 6 m s−1). Good high-frequency response was attained by the use of a very thin, bare wire probe of diameter 0.13 mm. Breaking waves were detected based on the energy in the 18–32 Hz frequency band. An appropriate threshold was found by a trial and error method. General agreement was found with visual observations. The temporal intermittency of wave breaking (fraction of time spent in breaking regions) was found to be 1.2%;however, the fraction of high-frequency (5–50 Hz) energy in those region was 12%. An argument is presented to show that the spatial intermittency and the spatial energy fraction should have the same values. The mean crest height of the breaking waves was found to be much lower than expected from theoretical considerations (by a factor of 4).

Non-Steady-State, Hot Wire, Thermal Conductivity Apparatus

A non-steady-state, hot wire apparatus has been constructed to measure the absolute or relative thermal conductivities of fluids. With the use of a four-lead bare wire probe, a precision of 0.1%, and an absolute accuracy of ± 1% have been obtained. A quartz-coated film sensor is used to obtain relative thermal conductivity data for salt water solutions with a precision of 1% and an over-all accuracy of ± 2%. A comparison is made with experimental data available from the literature. For toluene, a comparison is made with the extrapolated values of Poltz and Jugel from which errors due to radiation have been eliminated for the parallel plate apparatus. The values agree within experimental error.

Applications of the Electronic Probe to the Study of Turbulent Flames

Description is given of a negatively biased bare wire probe with suitable amplification and pulse-height discrimination to be used in detecting flame fronts in turbulent motion. Typical measurements are given of a Bunsen flame's most probable position, its rms displacement from this position, and its degree of continuity during unstable burning. The treatment of data and the precision and limitations of the method are discussed.