Thermal performance enhancement in tubes using helically twisted tape with alternate axis inserts

This article presents an investigation on heat transfer enhancement in a round tube inserted with a helically twisted tape. The effects of a helically twisted tape with alternate axis (HTT-A) on heat transfer, friction factor, and thermal performance factor behaviours are reported for the turbulent regime. HTT-A geometries are tape pitch to tube diameter, P/D = 1.0, 1.5, and 2.0; alternate length to pitch length, l/P = 1.0, 1.5, and 2.0; twisted length to tape width, y/W = 3.0; and tape width to tube diameter, w/D = 0.2. The experiment has been performed by varying the volumetric air flow rate in order to adjust Reynolds number ranging from 6 000 to 20 000. The wall of the testing tube is uniformly heated as a constant heat flux while the tests are covered with thermal insulations to reduce heat loss to surroundings. Thermal performance is evaluated by comparing the present experimental results with the results of the modified HTT-A and also those obtained from previous study (conventional helically twisted tape, HTT). The thermal performance of tested tube with HTT-A is evaluated to obtain the degree of heat transfer enhancement and friction factor induced by HTT-A with respect to the plain tube under the same test conditions. Evenly, it is interesting to observe that the tube with HTT-A consistently possesses higher heat transfer and thermal performance factor than those with the HTT around 14.1% and 1.9%, respectively. The HTT-A with the smaller pitch ratio and adjacent twist length provides higher heat transfer rate and friction factor than the one with larger pitch ratio and alternate length as a result of a larger contact surface area, stronger swirl intensity and, thus, better fluid mixing near the tube wall. In the range determined, the tubes with the largest pitch ratio (P/D = 2.0) and smallest alternate length (l/P = 1.0) give the highest thermal performance factor at around 1.35. In addition, the empirical correlations of the Nusselt number, friction factor, and thermal performance factor are also described.     

Tip-induced oxidative nano-machining of conducting diamond-like carbon (DLC)

Electrically conducting diamond-like carbon films have been nano-machined by local deposition of thermal energy at the tip-to-film point of contact; the process is implemented on an atomic force microscope platform. Features with linewidth resolution down to 20 nm have been demonstrated; lateral irregularities along edges were less than 5 nm, while the radius of curvature at the edges was less than 10 nm; and the slope of features was limited by the aspect ratio of the tip. The mechanism arises from prompt thermal oxidation by intermittent transfer of heat from an ohmically heated tip, where Fowler–Nordheim tunnelling is likely to play a role in completing an electrical circuit when the physical continuity of the thermal circuit is interrupted. When heat transfer is insufficient to ensure prompt oxidation, then formation of a metastable low-density carbon phase is found to take place. That phase will then, at room temperature and in the presence of oxygen, relax back to a normal higher density phase over a period of one hour. The many desirable physico-chemical properties of diamond-like carbon, in combination with the good spatial resolution of local probe methodology, suggest that the outcomes could have significant implications for next-generation nano-machine and nano-templating technologies. PACS 07.79.Lh; 81.16.Nd; 81.16.Pr; 81.16.Rf     

Very low temperatures and their applications in nuclear orientation

In NO-experiments at very low temperatures problems arise from the finite life-time of the nuclei and the radioactive self-heating of the source. Recent developments of top loading hyperfine enhanced nuclear demagnetization cryostats of small cooling power give fast access to temperatures around 1 mK. To reach lower temperatures the bottleneck of thermal contact must be overcome in order to cool samples which are themselves the main source of heat. In this context the limitations of nuclear demagnetization cryostats are discussed with respect to their application in brute force- and NQR/NMR-experiments.     

Renormalization group specific heat and magnetization of the Ising ferromagnet in cubic and hypercubic lattices

A real space renormalization group approach with two-terminal clusters is proposed for the calculation of the specific heat and spontaneous magnetization (for all temperatures) of the nearest-neighbour spin –1/2 Ising ferromagnet in simple cubic and hypercubic lattices. For arbitrary temperatures only small clusters (renormalization expansion factorb=2,3 ford=3 andb=2 ford=4) are considered: they lead to reasonable values for the critical points, exponents and amplitudes, to a thermal behaviour of the spontaneous magnetization which is already quite close to what is expected (from series results for example) but to a thermal behaviour of the specific heat which is not yet very close to what is expected. The global results improve whenb increases from 2 to 3. The drastic effect (on the specific heat) of an apparently innocuous approximation is exhibited. The discussion of theT0 andT limits is performed and theexact behaviours of the free and internal energies and the specific heat are obtained for sufficiently large values ofb andall dimensionalities.On leave of absence from Laboratorio de Física Teórica. Departamento de Física, Universidad Nactional de La Plata, CC No. 67 (1900), La Plata, Argentina.     

Thermodynamics of a quantum dissipative charged magneto‐oscillator

Quantum dissipative effect on the thermodynamics of an electron in the combined presence of a parabolic potential and a uniform (and homogeneous) magnetic field, is investigated here. Starting from the microscopic system plus bath model, we explicitly derive the thermodynamic properties using the reduced partition function of the system which is calculated using the imaginary time path integral method. The quantum heat bath we consider here is a structured heat bath whose spectral density corresponds to a structured thermal harmonic noise. All the statistical thermodynamic functions calculated do reconcile with the requirements of the fundamental axioms of physics. In particular, the specific heat and the entropy vanishes as the temperature approaches its absolute zero value, a necessity of the third law of thermodynamics. Moreover the specific heat satisfies classical equipartition theorem at high temperatures. The coefficients of the leading temperature dependent terms of the thermodynamic quantities depend only on the damping constant but not on other parameters of the bath spectral density, which is similar to the analysis based on the Drude bath spectral density. Our study facilitates the physics of small quantum systems, which are always in contact with some environments, at very low temperatures.

     

Thermodynamic Uncertainty Relations

Bohr and Heisenberg suggested that the thermodynamical quantities of temperature and energy are complementary in the same way as position and momentum in quantum mechanics. Roughly speaking their idea was that a definite temperature can be attributed to a system only if it is submerged in a heat bath, in which case energy fluctuations are unavoidable. On the other hand, a definite energy can be assigned only to systems in thermal isolation, thus excluding the simultaneous determination of its temperature. Rosenfeld extended this analogy with quantum mechanics and obtained a quantitative uncertainty relation in the form U (1/T) k, where k is Boltzmann's constant. The two extreme cases of this relation would then characterize this complementarity between isolation (U definite) and contact with a heat bath (T definite). Other formulations of the thermodynamical uncertainty relations were proposed by Mandelbrot (1956, 1989), Lindhard (1986), and Lavenda (1987, 1991). This work, however, has not led to a consensus in the literature. It is shown here that the uncertainty relation for temperature and energy in the version of Mandelbrot is indeed exactly analogous to modern formulations of the quantum mechanical uncertainty relations. However, his relation holds only for the canonical distribution, describing a system in contact with a heat bath. There is, therefore, no complementarily between this situation and a thermally isolated system.     

Miniaturized TEG with thermal generation of free carriers

The rising interest in low temperature heat energy conversion encourages the application of thermoelectric devices. However, conventional thermoelectric devices used in the Seebeck mode as thermoelectric generators have several shortcomings and thus are inefficient when used as a generator. Additionally, the high cost–power ratio of these modules anticipates the commercial success on a broad basis. One way to achieve better suited products is provided by miniaturization of thermoelectric devices in order to enable the use of mass production methods. But in small devices the contact effects become dominant and reduce the efficiency and power density considerably. We show that using pn‐junctions with thermal generation of free carriers offers the possibility to achieve better contact properties and thus higher efficiencies and power densities.

     

Simultaneous measurement of thermal diffusivity,thermal conductivity and specific heat by impulse-response photopyroelectric spectrometry

A novel photothermal technique is developed, which enables the simultaneous measurement of the thermal diffusivity , thermal conductivity , and the specific heat C of a sample. The technique is based on frequency-modulated time-delay photopyroelectric spectrometry (FM-TDPS), which consists of chirped laser excitation of the sample and detection of the thermal impulse response by a thin-film pyroelectric detector. No calibration is required for the measurements; absolute values for , , and C may be obtained without having to employ a reference sample. Results on superconducting YBa₂Cu₃O_7–x are reported for the temperature range 50–300 K; the values obtained compare favorably with reported measurements of , , and C for YBa₂Cu₃O_7–x , which previously required separate experiments for their determination.     

Thick gold-film deposition by high-repetition visible pulsed-laser chemical vapor deposition

A highly conductive gold film, over 10 m-thick with well-controlled linewidth, has been successfully deposited from dimethyl-gold-acetylacetonate and its fluorinated derivative by pyrolytic CVD (Chemical Vapor Deposition) with a high-repetition, visible, pulsed laser. The thermal damage to the polyimide substrate has been substantially suppressed by reducing the thermal diffusion length within 0.2 m in pulsed-laser-induced transient heating, in contrast to the cw laser-CVD scheme. Reproducible and low contact resistance as low as 0.5 between the written line and the existing gold line has been obtained. Sufficiently tough adhesion to polyimide has been observed for the deposit from dimethyl-gold-acetylacetonate. Reasonable agreement has been obtained between the observed deposition characteristics and analytical results for precursor supply rate and temperature increase during short-pulse irradiation.     

Temperature distribution in a film heated with a laser spot: Theory and measurement

For the measurement of the thermal capacity and the thermal conductivity of films, the thermal excitation of the sample is commonly performed by the absorption of light. This results in a spatial and temporal temperature distribution within the film. With a variety of methods static or dynamic temperature recordings are performed.Two problems with these methods are discussed, the calculation of the temperature distribution in the film and the measurement of the mean surface temperature of the film. An analytical solution of the heat conduction problem for a cylindrical geometry with any radial distribution of the absorbed light is given. Resistive bolometers are introduced for the measurement of the mean surface temperature of the film within a circular area. Experiments with a 25 m thick PVDF film give excellent agreement with the theoretical calculations within the modulation frequency range 10^–3 Hz to 10³ Hz, thus allowing a determination of various thermal parameters of the investigated film.     

Surface temperature measurements during ion bombardment

A copper constantan thin film thermocouple has been used to measure the surface temperature of a target during ion bombardment. Argon beams of 50–100 keV energy and 0–300 mwatt power have been used as heat source on a target in contact with a massive copper holder maintained at 80 or at 300K. The thermal contact has been changed from that ensured by the mechanical pressure of a metal spring to that obtained by gluing the target to the holder with a silver base paste. The surface temperature reaches a steady state value after about 150 s of bombardment and its maximum value depends on the beam power and on the quality of the thermal contact with the holder. Beam induced surface temperature increase as lar ge as 270°C are measured in the case of a bad thermal contact and for a beam power of 300 mwatt.     

Study of Ti/Au,Ti/Al/Au,and Ti/Al/Ni/Au ohmic contacts to <Emphasis Type="Italic">n</Emphasis>-GaN

The contacts of Ti/Au, Ti/Al/Au, and Ti/Al/Ni/Au films deposited on n-GaN were studied by current–voltage (I–V) and transmission-line-method measurements. The effect of annealing temperature on specific contact resistivity has been investigated by changing the annealing temperature from 400 to 900 °C. Ti/Al/Au and Ti/Al/Ni/Au films were superior to the bilayer (Ti/Au) in ohmic contact characteristics and thermal stability. The Ti/Al/Ni/Au composite showed the best thermal stability due to the fact that Ni plays a more important role than the alloy of Ti/Al in preventing the interdiffusion of Ti, Al, and Au. The lowest contact resistivity (10^-7cm²) to n-GaN was obtained for the Ti/Al/Ni/Au sample by short-time/high-temperature annealing. The formation mechanism of ohmic contacts to n-GaN is also discussed. PACS 73.40.Cg; 73.61.Ey     

Thermal conductivity of doped $\mathsf{La_2CuO_4}$ as an example for heat transport by optical phonons in complex materials

We investigate the phonon thermal conductivity of doped based on out-of-plane thermal conductivity measurements. When room temperature is approached the temperature dependence of strongly deviates from the T^-1-decrease which is usually expected for heat transport by acoustic phonons. Instead, decreases much weaker or even increases with rising temperature. Simple arguments suggest that such unusual temperature dependencies of are caused by heat transport via dispersive optical phonons.Received: 20 October 2003, Published online: 20 April 2004PACS: 66.70. + f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves - 74.72.Dn La-based cuprates - 44.10. + i Heat conduction     

Low temperature thermal properties of crystalline quartz after electron irradiation

Measurements of the temperature specific heat (between 0.12K and 8K) and thermal conductivity (between 0.5K and 20K) of crystalline -quartz after electron irradiation are reported. In the temperature region below 1K the specific heat is larger than in the unirradiated sample. This can be attributed to low energy excitations which are created during irradiation and which are associated with Al impurities. The thermal conductivity is reduced after irradiation. Below 4K the additional thermal resistivity varies asT ^–1.5. The phonon scattering by radiation-induced excitations in -quartz is weak compared to phonon scattering by two level systems (TLS) in vitreous silica.     

The thermal characteristics of “Se-As”-glasses in the neighbourhood of the transformation point

In this work the dependence of specific heat per unit volume, thermal conductivity and of thermal diffusivity on the temperature in amorphous specimen of selenium containing from 1 to 20 atomic percent of arsenic is studied. It has been found out that the admixture of arsenic considerably moves the transformation point-Tg-to higher values. It is shown that for definition of Tg it is advantageous to employ the extremes of the thermal diffusivity curve which is simply measured and has immediate connection with the microprocesses (with the mean free path of phonons). The changes of the specific heat per unit volume and of the thermal diffusivity in the point Tg are first growing, with increasing content of As, they reach a maximum in the neighbourhood of about 5 atomic percent of arsenic, and afterwards they are again decreasing. The measured curves are explained by the appearance of the additional thermal modes in the neighbourhood of the transformation point.Dúbravská cesta, Bratislava, Czechoslovakia.I thank Dr. J. Krempaský, C.Sc., for his valuable advices and remarks.     

Optimisation d'un cycle de Brayton moteur en contact avec des capacités thermiques finies

Optimization of a Brayton cycle engine in contact with fluid thermal capacities. Finite time thermodynamics studies have recently been devoted to all kind of cycles. Some works have been done on Brayton engine cycles.The proposed model is a synthesis of the preceding ones; this model completes the preceding papers, particularly regarding the two following aspects:

• 1.it takes into account the thermal losses occurring between heat source and sink, that are finite time reservoirs;
• 2.it influences the two limiting conditions for fluids at source and sink (four cases). The study is performed with partial and total regeneration, and without regeneration of heat. Influence of global internal irreversibility of the engine is also included in the sensitivity analysis of the proposed model.

Results are presented in a nondimensionnal form useful for design project. Limiting cases are enlightened by the proposed method, and allow us to disuss the whole set of previously obtained results.     

Discretization of the thermal excitation in highly excited matter

In this paper the thermal energy diffusion for quantum particles is described. The quantum heat transport equation is obtained. It is shown that, for a short-time thermal excitation (of the order of the relaxation time), the excited matter response is quantized on the different levels (atomic, nuclear, quark) with quantum thermal energy equalE ^atomic 9 eV,E ^nuclear 7 MeV, andE ^quark 139 MeV.     

Microstructure and current transport in Ti/Al/Ni/Au ohmic contacts to n-type AlGaN epilayers grown on Si(111)

The microstructure and the current transport in Ti/Al/Ni/Au ohmic contacts on AlGaN were investigated in this paper.Significant structural changes of the metal stack occurred upon annealing and ohmic contacts were obtained after thermal treatments above 700 C, with specific contact resistance values , which depend on the thickness of the Ti layer.Although the formation of a TiN interfacial layer is independent of the Ti thickness, different contact structures, i.e. the phases and grain location, were found as a function of the Ti layer, which were crucial for the nanoscale current transport through the contact stack. In particular, conductive atomic force microscopy combined with the resistivity measurement of the main phases formed upon annealing (AlNi, AlAu₄, Al₂Au) indicated that the low resistivity Al₂Au phase provides preferential conductive paths for the nanoscopic current flow through the contact.