236 research outputs found
Passive techniques for the enhancement of convective heat transfer in single phase duct flow
This review presents the main results of the experimental campaign on passive
techniques for the enhancement of forced convective single phase heat transfer in ducts,
performed in the last years at the Laboratory of the Industrial Engineering Department of the
University of Parma by the Applied Physics research group. The research was mainly focused
on two passive techniques, widely adopted for the thermal processing of medium and high
viscosity fluids, based on wall corrugation and
on
wall curvature
.
The innovative compound
heat transfer enhancement technique that couples together the effect of wall curvature and of
wall corrugation has been investigated as well. The research has been mainly focused on
understanding the causal relationship between the heat transfer surface modification and the
convection enhancement phenomenon, by accounting the effect of the fluid Prandtl number.
The pressure loss penalties were also evaluated. The principal results are presented and
discussed
PARAMETER ESTIMATION APPLIED TO THE HEAT TRANSFER CHARACTERISATION OF SCRAPED SURFACE HEAT EXCHANGERS FOR FOOD APPLICATIONS
A parameter estimation approach was applied to characterise the heat transfer of Scraped Surface Heat Exchangers (SSHEs) specifically designed for the food industry. It is difficult to apply the data available in the literature to SSHEs, due to the specificity of each product, thermal treatment and geometrical configuration, making the thermal design of these apparatuses critical. Therefore, it appears to be more useful to assess the methodology used to derive a proper heat transfer correlation than to assess the form of the heat transfer correlation itself, as the correlation often cannot be transferred to other heat exchangers, even those that belong to the same class.
This study enabled successful and robust estimation of the heat transfer correlation for the product side Nusselt number and the external side heat transfer coefficient; this approach differs from Wilson plot methods, as no assumption is made regarding the functional dependence of the external side heat transfer coefficient.
The procedure was validated through application to both synthetic data and experimental data acquired from a coaxial SSHE pilot plant for the treatment of highly viscous fluid foods.
The procedure was optimised with the aid of sensitivity and uncertainty analysis, which provided considerable insight into the problem.
The application to synthetic data demonstrated that under typical operating conditions, areas of insensitivity to certain parameters are present. The application to the experimental data acquired under both heating and cooling conditions confirmed that the measured values of the overall heat transfer coefficient can be used to estimate the secondary fluid heat transfer coefficient, as well as the power law dependence of the internal fluid Nusselt number on the rotational Reynolds number and the Prandtl number together with the multiplicative constant. The uncertainty analysis provided the confidence intervals associated with each estimated parameter, thereby enabling the quality and robustness of the resulting heat transfer correlations to be determined
Heat transfer delay method for the fluid velocity evaluation in a multi-turn pulsating heat pipe
A multi-turn closed loop pulsating heat pipe made of aluminium is tested in vertical bottom heated mode and
different condenser temperatures with the aim of providing quantitative information regarding its flow dynamics
through a novel post-processing technique on the local wall-to-fluid heat flux, evaluated within the adiabatic
section. The studied device is made of an annealed aluminium tube (inner/outer diameter: 3/5 mm), folded in 14
turns and partially filled with methanol (volumetric filling ratio: 50%). The aluminium channels are coated with
a high-emissivity opaque paint, thus allowing thermographic measurements on the outer wall by means of a
high-resolution medium wave infrared camera. The proposed method, named Heat Transfer Delay Method, is
validated by means of a dedicated experimental approach. Then, the acquired time-space temperature maps are
used as input data for the inverse heat conduction problem resolution approach to estimate the local convective
heat flux locally exchanged at the inner wall-fluid interface. The resulting wall-to-fluid heat fluxes are then post-
processed by applying the Heat Transfer Delay Method to the oscillatory and circulatory flow modes. The average
fluid velocity is assessed at varying working conditions during the circulatory flow, finding values up to 0.77 m/s
and 0.3 m/s for condenser temperature equal to 20 â—¦C and 10 â—¦ C, respectivel
Experimental analysis of the evaporation of a thin liquid film deposited on a capillary heated tube: estimation of the local heat transfer coefficient
The aim of this work is to estimate the local heat flux and heat transfer coefficient
for the case of evaporation of thin liquid film deposited on capillary heated channel: it plays a
fundamental role in the two-phase heat transfer processes inside mini-channels. In the present
analysis it is investigated a semi-infinite slug flow (one liquid slug followed by one single vapour
bubble) in a heated capillary copper tube. The estimation procedure here adopted is based on the
solution of the inverse heat conduction problem within the wall domain adopting, as input data,
the temperature field on the external tube wall acquired by means of infrared thermography
Thermal Performance Analysis of Triple Heat Exchangers via the Application of an Innovative Simplified Methodology
Despite the double tube heat exchangers, in the triple tube heat exchangers, there are three fluids, and the methodology based on the assessment of the logarithmic mean temperature difference is no longer applicable. Moreover, in triple tube heat exchangers, there are two overall heat transfer coefficients dependent on each other. As such, it is necessary to solve them simultaneously, thus making the evaluation of the thermal performance of triple tube heat exchangers more complex compared to double tube heat exchangers. Among the proposed approaches in the literature to solve this issue, one of the most powerful and commonly adopted in several engineering applications is the parameter estimation procedure. Nevertheless, for the specific implementation examined in our analysis, a thorough numerical model of the triple tube heat exchanger was required to apply the inverse procedure properly. Furthermore, it is mandatory to measure the temperature of the three fluids at the inlet and outlet sections. In so doing, the inverse procedure can be successfully applied to the characterisation of triple tube heat exchangers tested in well-equipped research labs; however, its application to heat transfer devices operating in industrial facilities can be difficult. In order to overcome this limitation, an innovative parameter estimation technique that enables the evaluation of the thermal performance of this type of heat transfer devices is presented. The suggested methodology is based on a simple model of the triple tube heat exchanger in which an equivalent double tube heat exchanger is considered, thus requiring only four temperature measurements. The results obtained by applying this simplified methodology are numerically validated and compared to those obtained using a comprehensive mode
Thermal characterisation of Triple Concentric Tube Heat Exchangers by applying parameter estimation: Direct problem implementation
Heat transfer enhancement in heat exchangers's design represents a key technological challenge because of the increase in the cost of energy and raw materials. A promising technology is represented by the triple tube heat exchangers, in which the heat transfer is enhanced in comparison with the traditional double tube heat exchangers, due to the larger heat transfer area per unit length. Among the different methodologies that can be adopted to assess the performance of the triple tube heat exchangers, parameter estimation procedure represents a promising tool, since it has been successfully applied in many disciplines of engineering. To apply this inverse technique, it is mandatory defining the direct problem, which for the issue here addressed allows evaluating the outlet temperatures of the fluids flowing in the heat exchanger. Since in a triple tube heat exchanger there are three fluids, the approach based on the evaluation of the logarithmic mean temperature difference is no longer valid and an alternative procedure has to be followed. In the present analysis a numerical model for the performance evaluation of triple tube heat exchangers is presented. The validation of the proposed numerical model, carried out by adopting the analytical model available in literature, highlights that the model can be considered accurate and reliable. Moreover, the computational time required to solve the set of equations is very limited
Thermal characterization of intumescent fire retardant paints
Intumescent coatings are now the dominant passive fire protection materials used in industrial and commercial buildings. The coatings, which usually are composed of inorganic components contained in a polymer matrix, are inert at low temperatures and at higher temperatures, they expand and degrade to provide a charred layer of low conductivity materials. The charred layer, which acts as thermal barrier, will prevent heat transfer to underlying substrate. The thermal properties of intumescent paints are often unknown and difficult to be estimated since they vary significantly during the expansion process; for this reason the fire resistance validation of a commercial coatings is based on expensive, large-scale methods where each commercial coating-beam configuration has to be tested one by one. Adopting, instead, approaches based on a thermal modelling of the intumescent paint coating could provide an helpful tool to make easier the test procedure and to support the design of fire resistant structures as well. The present investigation is focused on the assessment of a methodology intended to the restoration of the equivalent thermal conductivity of the intumescent layer produced under the action of a cone calorimetric apparatus. The estimation procedure is based on the inverse heat conduction problem approach, where the temperature values measured at some locations inside the layer during the expansion process are used as input known data. The results point out that the equivalent thermal conductivity reached by the intumescent material at the end of the expansion process significantly depends on the temperature while the initial thickness of the paint does not seem to have much effect
Thermal Performance Investigation by Infrared Analysis of Mini Pulsating Heat Pipe
A promising solution in the field of passive two-phase heat transfer devices is represented by Pulsating Heat Pipes (PHPs). They are undoubtedly appealing due to the high heat transfer capability, efficient thermal control, adaptability and low cost. In the last years they are raising concern for space applications that are characterised by extreme environmental conditions, strictly constrains in terms of compactness, reliability and the need to dissipate efficiently heat in microgravity conditions. In this study, the thermal performance of oscillating heat pipes that consists of extra-thin metallic pipes are investigated: the adoption of metallic pipes with an inner diameter less than 0.4 mm permits to couple flexibility and compactness with high heat transfer performance. HFC-134a is used as working fluid. Many authors have investigated the pulsating behaviour of this type of heat transfer devices only considering the average temperature of the evaporator and condenser. In this work, to deeply investigate the oscillating behaviour of the proposed PHP, it is adopted an approach based on the study of the local temperature distributions on the wall of the PHP, acquired with a high-speed and high-resolution infrared camera. The local analysis of the temperature trends is of fundamental importance in the understanding of the complex phenomena that govern the pulsating field
Design and implementation of a modulating test plant to assess the performance of innovative cross-flow heat recovery units for air conditioning system: Preliminary results
Nowadays global warming has increased consciousness of the dangers of energy wastefulness: in the last 50 years the temperature of the Earth's surface rose by approximately 1 °C. The building sector is responsible for a very high percentage of world carbon emissions and with the increasing of the request for comfort, heating, ventilation and air conditioning, buildings energy consumption is rapidly growing. Consequently, it appears fundamental the role played by the improvement of buildings energy performance within global policies of emissions reduction. In this context an increasing attention is given to the energy waste reduction in tertiary sector: bars, offices, restaurants, meetings, shops, school buildings, gyms and in general in the buildings in which the minimization of the energy dissipation is requested. The present study is part of the NANOFANCOIL project (POR-FESR 2014-2020): one of the objectives was the design and implementation of a modulating test plant to assess the performance of innovative cross-flow heat recovery units for air conditioning system. This experimental setup was mainly composed by two climatic chambers that enabled to simulate the environmental conditions of interest. The temperature could vary from -20°C to 0°C and from 10°C to 30°C for the cold and the hot chamber, respectively (i.e. the outdoor and the indoor environment). Moreover, the climatic chamber that simulates the indoor environment could be controlled also in terms of humidity thanks to a steam humidifier that guaranteed 5 kg/h of vapour. Preliminary results about the heat transfer behaviour of a cross-flow heat recovery unit (air-to-air) for controlled mechanical ventilation systems that employs mini-channels in order to increase performance and reduce size and costs are presented here
Investigation of the effect of cylindrical insert devices on laminar convective heat transfer in channel flow by applying the Field Synergy Principle
The Field Synergy Principle is widely applied to the evaluation of the convective heat transfer mechanism. In fact, as highlighted in literature, the evaluation of the synergy between the velocity and the temperature gradient vectors could provide a better insight on the local convective heat transfer mechanism. In this paper, the field synergy approach is adopted to numerically investigate the fluid dynamic and thermal behaviour of a fully developed flow between parallel plates with asymmetric heating, when cylindrical inserts are present. To better evaluate the influence of the inserts on the convective heat transfer mechanism, different values of the insert diameter are considered, for a given pitch value. The numerical results in terms of Nusselt number point out that the convective heat transfer coefficient decreases as the insert diameter increases. The Field Synergy Principle allows to explain the cause of the convective heat transfer reduction identifying the regions in which the heat transfer mechanism is ineffective: the extent of these areas increases as the insert diameter increases
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