Giovanni Tanda

Heat transfer in rectangular channels with transverse and V-shaped broken ribs

Repeated ribs are used on heat exchange surfaces to promote turbulence and enhance convective heat transfer. Applications include fuel rods of gas-cooled nuclear reactors, inside cavities of turbine blades, and internal surfaces of pipes used in heat exchangers. Despite the great number of literature papers, only few experimental data concern detailed distribution of the heat transfer coefficient in channels with rib turbulators. This issue was tackled by means of the steady-state liquid crystal thermography: a pre-packaged liquid crystal film was glued onto the heated surface, and the colour map was taken by a video camera at the steady state of a given experiment. After calibration tests to assess the colour–temperature relationship had been performed, local heat transfer coefficients were obtained by applying custom-made software to process the digitised colour images. Liquid crystal thermography was applied to the study of heat transfer from a rectangular channel (width-to-height ratio equal to five) having one surface heated at uniform heat flux and roughened by repeated ribs. The ribs, having rectangular or square sections, were deployed transverse to the main direction of flow or V-shaped with an angle of 45 or 60 deg relative to flow direction. The effect of continuous and broken ribs was also considered. Local heat transfer coefficients were obtained at various Reynolds numbers, within the turbulent flow regime. Area-averaged data were calculated in order to compare the overall performance of the tested ribbed surfaces and to evaluate the degree of heat transfer enhancement induced by the ribs with respect to the smooth channel.

Heat transfer and pressure drop in a rectangular channel with diamond-shaped elements

Abstract Heat transfer and pressure drop experiments were performed for a rectangular channel equipped with arrays of diamond-shaped elements. Both in-line and staggered fin arrays were considered, for values of the longitudinal and transverse spacings, relative to the diamond side, from 4 to 8 and from 4 to 8.5, respectively. The height-to-side ratio of the diamonds was 4.0. Liquid crystal thermography was used to determine the heat transfer coefficients on the surface of the channel (endwall) on which the fins were mounted. Local variations in heat transfer coefficients induced by the arrangements of the diamond-shaped elements were measured and discussed. Correlations giving the average Nusselt number for each fin configuration as a function of the Reynolds number were developed. Thermal performance comparisons with data for a rectangular channel without fins showed that the presence of the diamond-shaped elements enhanced heat transfer by a factor of up to 4.4 for equal mass flow rate and by a factor of up to 1.65 for equal pumping power.

Natural convection heat transfer in vertical channels with and without transverse square ribs

Abstract Experiments were performed to determine heat transfer data for the natural convective flow of air in vertical channels with one surface roughened by transverse square ribs and the opposite surface smooth. Uniform wall temperature conditions were imposed on the ribbed side, while the smooth side remained unheated. Additional experiments were carried out in vertical channels without ribs, under the same channel geometry and thermal conditions. A schlieren optical technique was used to reconstruct the thermal field and to obtain distributions of heat transfer coefficients. The presence of ribs was found to alter heat transfer considerably, causing thermally inactive regions just upstream and downstream of each protrusion. Consequently, the heat transfer performance of the ribbed channel turned out to be lower than that of the corresponding smooth channel.

Study of free convection frost formation on a vertical plate

Abstract Processes involving heat transfer from a humid air stream to a vertical plate, with simultaneous deposition of frost, are of great importance in a variety of refrigeration equipment. In this work, frost growth on a vertical plate in free convection has been experimentally investigated. The plate, cooled by the internal circulation of glycol, was placed in a vertical channel open at the top and bottom in order to permit the natural circulation of ambient air. The cold plate temperatures were varied in the −13 to −4 °C range, while the relative humidity and temperature of the ambient air were taken in the 31–58% and 26–28 °C ranges, respectively. Measured quantities (frost thickness, frost surface temperature, deposited mass of frost, heat flux at the plate/frost interface) were compared with results given by a simple model based on standard relationships (for heat and mass transfer coefficients, frost density and thermal conductivity) reported in the literature.

An experimental investigation of forced convection heat transfer in channels with rib turbulators by means of liquid crystal thermography

Abstract An optical technique for measuring local heat transfer coefficients in enhanced heat transfer channels is discussed. The technique is based on the use of thermochromic liquid crystals applied to the test surface. The two-dimensional temperature distribution can be easily visualised as a colour distribution. The colour pattern is observed by a CCD videocamera and the recorded image is digitised and converted from the RGB (red, green, blue) to the HSI (hue, saturation, intensity) domain. Among these new parameters, the hue can be directly related to surface temperature through a linear and repeatable relationship, determined by means of previous calibration experiments. If a uniform heat flux condition is applied to the test surface, the pattern of the heat transfer coefficient can be obtained. This optical technique was applied to the study of forced convection heat transfer characteristics in a rectangular channel with rib turbulators mounted on one of the surfaces. Continuous and broken parallel ribs, deployed normal to the main direction of air flow, were considered, for different values of the geometric parameters. The range of variation of the Reynolds number, based on the hydraulic diameter of the channel, was 8000–35,000.

Measurement of Total Hemispherical Emittance and Specific Heat of Aluminum and Inconel 718 by a Calorimetric Technique

An apparatus for the measurement of the total hemispherical emittance and specific heat of metals has been developed. The measurement principle is based on the calorimetric technique: the sample, heated by Joule effect and placed in a vacuum chamber; exchanges radiative heat transfer with the walls of the container, kept at a relatively low temperature. Emittance is deduced from the radiative heat transfer laws at the steady state. When the heating power is switched off the specific heat of the sample can be recovered from the time history of the sample temperature during the cooling transient. Measurements have been performed on samples of aluminum Anticorodal alloy and Inconel 718 alloy under different surface conditions in the 350-635 K range.

Forced Convection Heat Transfer in Channels With Rib Turbulators Inclined at 45 deg

Local and average Nusselt numbers and friction factors are presented for rectangular channels with an aspect ratio of 5 and angled rib turbulators inclined at 45 deg with parallel orientations on one and two surfaces of the channel. The convective fluid was air, and the Reynolds number varied from 9000 to 35,500. The ratio of rib height to hydraulic diameter was 0.09, with the rib pitch-to-height ratio equal to 13.33 or 6.66. Experiments were based on the use of heating foils (for the attainment of uniform heat flux condition) and of the steady-state liquid crystal thermography (for the identification of isotherm lines and the reconstruction of local heat transfer coefficient). Local results showed quasiperiodic profiles of Nusselt number in the streamwise direction, whose features were strongly affected by the value of rib pitch and by the spanwise coordinate. For all the investigated geometries a heat transfer augmentation, relative to the fully developed smooth channel, was found; when inclined rib turbulators were placed on two opposite surfaces of the channel, the full-surface Nusselt number was higher (by 10–19%) than that for the one-ribbed wall channel, but pressure drop penalties also increased by a factor of about 3. For both the one- and two-ribbed wall channels, the best heat transfer performance for a constant pumping power, in the explored range of Reynolds number, was generally achieved by the larger rib pitch-to-height ratio (=13.33).

Comparative measurements of natural convection heat transfer in channels by holographic interferometry and schlieren

In this work, natural convection heat transfer in vertical channels is experimentally investigated by applying different optical techniques, namely holographic interferometry and schlieren. Both these techniques are based on the temperature dependence of the air refractive index but they detect different optical quantities and their use involves different instrumentation and optical components. Optical methods, non-intrusive in nature, are particularly suitable for the visualization of flow and thermal fields as witnessed by their increasing use in a range of scientific and engineering disciplines; for this reason, the introduction of these experimental tools into a laboratory course can be of high value. Physics and engineering students can get familiarized with optical techniques, grasp the basics of thermal phenomena, usually elusive, which can be more easily understood if they are made visible, and begin to master digital image analysis, a key skill in laboratory activities. A didactic description of holographic interferometry and schlieren is provided and experimental results obtained for vertical, smooth and rib-roughened channels with asymmetrical heating are presented. A comparison between distributions of the local heat transfer coefficient (or its dimensionless counterpart, the Nusselt number) revealed good agreement between the results separately obtained by the two techniques, thus proving their suitability for investigating free convection heat transfer in channels.

Second Law Performance Analysis for Offset Strip-Fin Heat Exchangers

Abstract A design procedure for offset strip-fin (OSF) heat exchangers is proposed, on the basis of a minimum-entropy-production evaluation criterion. Proper dimensionless operational and geometric-parameters are introduced, and the thermodynamic impact of the OSF augmentation technique is Analyzed, for assigned heat exchanger working conditions, using available correlations for heat transfer and fluid friction coefficients. The proposed method allows the designer to determine the most suitable geometric configuration in order to achieve—under given operational constraints—the minimum dissipation effects of OSF heat exchangers.

Experiments on natural convection from two staggered vertical plates

Abstract The thermal field and the heat transfer characteristics of a system consisting of two staggered vertical plates cooled by air in free convection were experimentally studied. The parameters investigated included the interplate spacing, the magnitude of the vertical stagger, and the Rayleigh number based on the overall convective heat flux from each plate. The experiments were performed in air. The schlieren optical technique was employed to obtain the thermal field around the plates and the local heat transfer coefficients along the vertical sides of plates. Staggering was found to markedly affect the local heat transfer characteristics of the facing sides of the plates when the interplate distance was relatively small. In general, the Nusselt number averaged on the inner face of the lower plate was enhanced (up to over 40%) compared with that for the case of the unstaggered plate channel. Conversely the mean Nusselt number on the facing side of the upper plate was reduced (up to 15%).