High-efficiency heat transfer devices by innovative manufacturing techniques

In the present thesis, novel methods devoted to develop high heat transfer efficiency devices have been presented. These methods rely on both novel manufacturing techniques, belonging to the class of additive manufacturing (AM), and thermal and fluid-dynamics studies and optimization procedures. As a first result, optimization of a traditional heat exchanger from a real application, i.e. million of units produced per year, is presented; That is manufactured by extrusion. A thermal fluid-dynamic model is experimentally validated (from an industrial experimental test rig) and used for optimization purposes. Results demonstrate there is room for efficiency optimization even in well established heat transfer devices configurations based on traditional manufacturing techniques. Then, an experimental rig for ''in house'' thermal characterization is designed. It guarantees high precision measurement of small convective heat fluxes (forced air) on enhanced solutions investigated hereinafter, namely micro-structured surfaces and small heat transfer devices. To deal with that challenge, an innovative convective heat flux sensor is developed. That exploits the concept of thermal guard to avoid any spurious perturbation between the flow field and investigated surfaces, while it allows to cancel out terms due to spreading conduction phenomenon. Results demonstrate remarkable accuracy in direct measurement of convective heat fluxes through this novel concept. Relying on the proposed experimental rig, various methods for enhanced convective heat transfer are experimentally investigated. Firstly, regular patterns of micro-protrusions are studied. Effect of fluid-dynamics and geometrical length on heat transfer performances are discussed. More important, they have been applied to develop an optimization procedure tailored to deal with AM techniques. Results from both experimental investigation and optimization procedure suggest the existence of an optimal value of protrusion height, that maximize performance-to-cost ratio for patterns made by AM. Then, surface roughness of components built by DMLS has been investigated as an augmentation heat transfer technique. Surface roughness is controlled varying DMLS process parameters and its effect on convective heat transfer is measured. The results demonstrate a remarkable enhancement in convective heat transfer due to DMLS artificial roughness, in the investigated configurations. That preliminary study unveils the potential of AM artificial roughness as an heat transfer enhancement techniques. It has been considered, by academic and industrial institutions, as an important step towards development of next generation gas turbine components and electronic cooling devices. Finally, extreme flexibility in shape of parts built by DMLS is exploited to design and fabricate in one step an unconventional heat transfer device, called Pitot heat exchanger. Enhanced heat transfer efficiency is achieved, with regard to standard heat exchangers. Nevertheless, the most important achievement has been to highlight unusual morphologies allowed by AM can pave the way to revolutionary changes in conceiving and designing heat transfer components

Convective heat transfer enhancement by diamond shaped micro-protruded patterns for heat sinks: Thermal fluid dynamic investigation and novel optimization methodology

In the present work, micro-protruded patterns on flush mounted heat sinks for convective heat transfer enhancement are investigated and a novel methodology for thermal optimization is proposed. Patterned heat sinks are experimentally characterized in fully turbulent regime, and the role played by geometrical parameters and fluid dynamic scales are discussed. A methodology specifically suited for micro-protruded patterns optimization is designed, leading to 73 % enhancement in thermal performance respect to commercially available heat sinks, at fixed costs. This work is expected to introduce a new methodological approach for a more systematic and efficient development of solutions for electronics cooling

Micro-structured rough surfaces by laser etching for heat transfer enhancement on flush mounted heat sinks

Abstract. The aim of this work is to improve heat transfer performances of ush mounted heat sinks used in electronic cooling. To do this we patterned 1.23 cm2 heat sinks surfaces by micro- structured roughnesses built by laser etching manufacturing technique, and experimentally measured the convective heat transfer enhancements due to dierent patterns. Each roughness diers from the others with regards to the number and the size of the micro-ns (e.g. the micro- n length ranges from 200 to 1100 m). Experimental tests were carried out in forced air cooling regime. In particular fully turbulent ows (heating edge based Reynolds number ranging from 3000 to 17000) were explored. Convective heat transfer coecient of the best micro-structured heat sink is found to be roughly two times compared to the smooth heat sinks one. In addition, surface area roughly doubles with regard to smooth heat sinks, due to the presence of micro-ns. Consequently, patterned heat sinks thermal transmittance [W/K] is found to be roughly four times the smooth heat sinks one. We hope this work may open the way for huge boost in the technology of electronic cooling by innovative manufacturing techniques

Efficient steam generation by inexpensive narrow gap evaporation device for solar applications

Technologies for solar steam generation with high performance can help solving critical societal issues such as water desalination or sterilization, especially in developing countries. Very recently, we have witnessed a rapidly growing interest in the scientific community proposing sunlight absorbers for direct conversion of liquid water into steam. While those solutions can possibly be of interest from the perspective of the involved novel materials, in this study we intend to demonstrate that efficient steam generation by solar source is mainly due to a combination of efficient solar absorption, capillary water feeding and narrow gap evaporation process, which can also be achieved through common materials. To this end, we report both numerical and experimental evidence that advanced nano-structured materials are not strictly necessary for performing sunlight driven water-to-vapor conversion at high efficiency (i.e. ≥85%) and relatively low optical concentration (≈10 suns). Coherently with the principles of frugal innovation, those results unveil that solar steam generation for desalination or sterilization purposes may be efficiently obtained by a clever selection and assembly of widespread and inexpensive materials

Passive heat transfer enhancement by 3D printed Pitot tube based heat sink

3D printing, also referred to as Additive Manufacturing - AM in case of metal materials, allows to fabricate complex shaped parts and devices in a single step. Extreme flexibility of AM techniques could pave the way to a revolution in conceiving heat transfer devices in the near future. Along this way, we designed and fabricated by AM an innovative heat sink incorporating Pitot tubes for realizing passive heat transfer enhancement. Preliminary tests show that the proposed heat sink allows up to 98% heat transfer augmentation, as compared to conventional heat sinks. We hope this study will help in encouraging the community to explore novel approaches thus moving towards the design of new devices fully exploiting the 3D printing flexibility in the field of thermal engineering

Identification of long non-coding transcripts with feature selection: a comparative study

Table S4. List of features ranked by each algorithm in each species. (XLS 63 kb

Unshrouded plate fin heat sinks for electronics cooling: Validation of a comprehensive thermal model and cost optimization in semi-active configuration

Plate Fin Heat Sinks (PFHS) are among the simplest and most widespread devices for electronics cooling. Because of the many design parameters to be considered, developing both cost and thermal effective PFHS is a critical issue. Here, a novel thermal model of PFHS is presented. The model has a broad field of applicability, being comprehensive of the effects of flow bypass, developing boundary layers, fin efficiency and spreading resistance. Experiments are then carried out to validate the proposed thermal model, and its good accuracy is demonstrated. Finally, an optimization methodology based on genetic algorithms is proposed for a cost-effective selection of the design parameters of PFHS, which is particularly effective with semi-active configurations. Such an optimization methodology is then tested on a commercial heat sink, resulting in a possible 53% volume reduction at fixed thermal performances

A sensor for direct measurement of small convective heat fluxes: Validation and application to micro-structured surfaces

A sensor for measuring small convective heat flows (<0.2 W/cm^2) from micro-structured surfaces is designed and tested. This sensor {exploits the notion of thermal guard and is purposely designed} to deal with metal samples made by additive manufacturing, {such as} direct metal laser sintering (DMLS). For validation purposes, we utilize both experimental literature data and a computational fluid dynamic (CFD) model: Maximum and average deviations from CDF model in terms of the Nusselt number are on the order of +/- 13.7 % and +/- 6.3 %, respectively while deviations from literature data are even smaller. Similar works in the literature often have the necessity of maintaining one-directional heat flows along the main dimension of a conducting bar using insulating materials. Such an approach can be critical for small fluxes due to the curse of heat conduction losses along secondary directions. As a result, it is necessary to estimate those secondary fluxes (e.g. by numerical models), thus making the measurement difficult and indirect. On the other hand, depending on the manufacturing accuracy, the present sensor enables to practically reduce at will those losses, with direct measurement of the heat flux. To our knowledge, the adoption of thermal guard is not a common practice in convective heat transfer, especially when local measurements are of interest. We hope that this study may (i) shed light on the usefulness of the approach in this field; and (ii) provide an effective tool for future investigation on electronic cooling and convective heat transfer enhancement by micro-/nano-structured surfaces. Owing to a number of features of the proposed device, we suggest that it can be prospectively utilized in the near future (i) for industrial applications (due to simplicity and robustness of the design); (ii) for high temperature measurements (unlike foil sensors, no delamination issues can be experienced); (iii) in the context of micro-electromechanical systems (MEMS) (easy to miniaturize)

Rough surfaces with enhanced heat transfer for electronics cooling by direct metal laser sintering

Experimental evidences are reported on the potential of direct metal laser sintering (DMLS) in manufacturing flat and finned heat sinks with a remarkably enhanced convective heat transfer coefficient, taking advantage of artificial roughness in fully turbulent regime. To the best of our knowledge, this is the first study where artificial roughness by DMLS is investigated in terms of such thermal performances. On rough flat surfaces, we experience a peak of 73 % for the convective heat transfer enhancement (63 % on average) compared to smooth surfaces. On rough (single) finned surfaces, the best performance is found to be 40 % (35 % on average) compared to smooth finned surface. These results refer to setups with Reynolds numbers (based on heated edge) within 3,500 < Re_L < 16,500 (corresponding to 35,000 < Re_D < 165,000 in terms of Reynolds number based on hydraulic diameter). Experimental data are obtained by a purposely developed sensor with maximum and mean estimated tolerance intervals of +/- 7.0 % and +/- 5.4 %, respectively. Following the idea by Gioia et al. [Phys. Rev. Lett. 96 (2006) 044502], we propose that heat transfer close to the wall is dominated by eddies with size depending on the roughness dimensions and the viscous (Kolmogorov) length scale. An excellent agreement between the experimental data and the proposed analytical model is finally demonstrated