Hydrophilic and Hydrophobic Nanostructured Copper Surfaces for Efficient Pool Boiling Heat Transfer with Water, Water/Butanol Mixtures and Novec 649

Increasing heat dissipation requirements of small and miniature devices demands advanced cooling methods, such as application of immersion cooling via boiling heat transfer. In this study, functionalized copper surfaces for enhanced heat transfer are developed and evaluated. Samples are functionalized using a chemical oxidation treatment with subsequent hydrophobization of selected surfaces with a fluorinated silane. Pool boiling tests with water, water/1-butanol mixture with self-rewetting properties and a novel dielectric fluid with low GWP (Novec™ 649) are conducted to evaluate the boiling performance of individual surfaces. The results show that hydrophobized functionalized surfaces covered by microcavities with diameters between 40 nm and 2 µm exhibit increased heat transfer coefficient (HTC; enhancements up to 120%) and critical heat flux (CHF; enhancements up to 64%) values in comparison with the untreated reference surface, complemented by favorable fabrication repeatability. Positive surface stability is observed in contact with water, while both the self-rewetting fluids and Novec™ 649 gradually degrade the boiling performance and in some cases also the surface itself. The use of water/1-butanol mixtures in particular results in surface chemistry and morphology changes, as observed using SEM imaging and Raman spectroscopy. This seems to be neglected in the available literature and should be focused on in further studies

Structured copper surfaces for enhanced nucleate boiling heat transfer

V delu obravnavamo vpliv mehanskega in laserskega strukturiranja bakrenih vzorcev na izboljšanje prenosa toplote pri mehurčkastem vrenju vode v bazenu. Vzorce smo mehansko obdelali z brušenjem ali peskanjem oz. lasersko strukturirali z namenom izdelave mikrojamic ali oksidne plastina eni površini smo po vzoru literature izdelali mikrokanale. Površine smo okarakterizirali z uporabo meritev topografije, izdelavo SEM posnetkov, EDS analizo kemijske sestave in meritvami kota omočenja. Na merilni progi lastne konstrukcije smo izvedli eksperimente z nasičenim vrenjem redestilirane vode v bazenu, pri čemer smo zabeležili vrelne krivulje posameznih površin pred prvim nastopom kritične gostote toplotnega toka in po njem ter izračunali koeficiente toplotne prestopnosti. Dosegli smo kritične gostote toplotnega toka do 1220 kW m-2 na mehansko obdelanih površinah, do 1580 kW m-2 na lasersko strukturiranih površinah in 1897 kW m-2 na površini z mikrokanali, kar predstavlja 46 %, 89 % oz. 127 % izboljšanje napram referenčni površiniizboljšanje koeficienta toplotne prestopnosti v primerjavi z referenčno površino znaša do 159 % (mehanska obdelava), 174 % (lasersko strukturiranje) oz. 296 % (mikrokanali). Ugotovili smo, da lasersko strukturiranje zagotavlja stabilne površine, ki se med zaporednimi meritvami minimalno spreminjajo. Prav tako smo opazili pojav spremembe površinske kemije kot posledice nastopa kritične gostote toplotnega toka, kar je imelo velik vpliv na prenos toplote z vrenjem.In the present work, the influence of mechanical and laser structuring of copper samples on enhanced nucleate pool boiling heat transfer is investigated. Samples were either mechanically treated using sanding or sandblasting or laser structured to produce microcavities or a surface oxide layermicrochannels were machined onto one sample. Characteristics of surfaces were determined through the use of topography measurement, SEM imaging, EDS surface chemical composition analysis and contact angle measurement. A pool boiling experimental setup of our own design was used to determine boiling heat transfer characteristics of structured surfaces during saturated boiling of twice-distilled water. Boiling curves were recorded before and after the first onset of critical heat flux and heat transfer coefficients were calculated. The following critical heat flux was reached: up to 1220 kW m-2 on mechanically treated surfaces, up to 1580 kW m-2 on laser treated surfaces and 1897 kW m-2 on the surface with microchannelsthis represents 46 %, 89% and 127 % improvement in regards to the reference surface, respectively. Heat transfer coefficient improvement ranged from 159 % (mechanical treatment) and 174 % (laser treatment) to 296 % (microchannels). It was determined that laser structuring produces stable surfaces with minimal deviation between consecutive measurements. Additionally, a change of surface chemistry was recorded after the first onset of the critical heat flux, which had a large influence on boiling heat transfer

Hybrid structured surfaces for enhanced nucleate boiling heat transfer

V delu obravnavamo prenos toplote pri mehurčkastem vrenju in njegovo izboljšanje oz. intenzifikacijo preko modifikacije vrelne površine in njene interakcije z delovnim fluidom. V ta namen predstavljamo razvoj hibridnih strukturiranih površin, ki promovirajo mehurčkasto vrenje. Za funkcionalizacijo površin vzorcev iz aluminija, bakra in titana smo uporabili predobdelavo s kemičnim ali laserskim strukturiranjem in nanos hidrofobnega premaza v obliki fluoriranega silana. Na podlagi analize literature, v kateri obstaja velik raztros referenčnih vrednosti kritične gostote toplotnega toka kot posledica različnih metodologij procesiranja izmerkov, smo razvili novo merilno progo za spremljanje procesa vrenja z nizko merilno negotovostjo. Na razvitih površinah smo ovrednotili prenos toplote pri nasičenem vrenju vode pri atmosferskem tlaku, pri čemer smo zabeležili vrednosti kritične gostote toplotnega toka do 1943 kW m-2 (> 100 % izboljšanje v primerjavi z neobdelano površino) in koeficiente toplotne prestopnosti do 304,7 kW m-2 K-1 (> 500 % izboljšanje). Pokazali smo, da se zaradi večkratnega nastopa kritične gostote toplotnega toka ali dolgotrajnega vrenja spremenijo morfološke in kemične lastnosti vrelne površine, kar vpliva na prenos toplote pri vrenju. Ovrednotenje vrenja samoomočljivih fluidov na razvitih hibridnih površinah je pokazalo, da se parametri prenosa toplote v splošnem poslabšajo v primerjavi z vrenjem čiste vode na istih površinah, še vedno pa so zagotovljeni višji koeficienti toplotne prestopnosti kot pri vrenju istih fluidov na neobdelanih površinah. Rezultati doktorskega dela kažejo, da primerno razplinjene površine z nizko površinsko energijo in mikrostrukturo, ki promovira nukleacijo, zagotavljajo velike izboljšave parametrov prenosa toplote pri mehurčkastem vrenju, kar je v nasprotju s trenutno uveljavljenim prepričanjem, da takšne površine, ki na makroskali izkazujejo superhidrofobnost, niso primerne za izboljšanje prenosa toplote pri vrenju.This work deals with nucleate boiling heat transfer and its enhancement through modification of the boiling surface and the interaction of the latter with the working fluid. To this effect, we present the development of hybrid structured surfaces, which promote nucleate boiling. Surfaces of aluminum, copper and titanium samples are functionalized using chemical or laser surface texturing and subsequent hydrophobization through application of a fluorinated silane. Analysis of reference critical heat flux values from literature shows that considerable scatter results from different methodologies of processing measured values. This was used to develop a new experimental setup for boiling performance evaluation with reduced measurement uncertainty. Boiling heat transfer was evaluated on developed surfaces using saturated pure water at atmospheric pressure. Developed surfaces exhibited critical heat flux values of up to 1943 kW m-2 (> 100 % enhancement over the reference surface) and heat transfer coefficients of up to 304,7 kW m 2 K-1 (> 500 % enhancement). We have shown that repeated onset of critical heat flux or long-term boiling on the surface changes morphological and chemical properties of the boiling surface, which affects boiling heat transfer. Boiling of self-rewetting fluids on developed hybrid structured surfaces resulted in deterioration of heat transfer intensity compared to boiling of pure water on the same surfaces, although heat transfer is still enhanced compared to boiling of these fluids on untreated surfaces. The results of the doctoral thesis demonstrate that properly degassed surfaces with low surface energy and nucleate-boiling-promoting microstructure provide substantial enhancements of nucleate boiling heat transfer parameters, which challenges the currently established sentiment that such (macroscopically superhydrophobic) surfaces are unsuitable for boiling heat transfer enhancement

Experimental and numerical heat transfer analysis of heat-pipe-based CPU coolers and performance optimization methodology

This paper presents an experimental and numerical heat transfer analysis of heat-pipe-based CPU coolers and a performance optimization methodology. The first part of the study focuses on the performance of two commercial HP-based CPU coolers under inclination angles of 0°, 90° and 180°. The results show that the 90° orientation provides the best thermal performance. The influence of heat pipe orientation on the performance of the entire system is obscured due to the much higher thermal resistance on the air-side of the cooler. A fourfold increase in air volumetric flow rate has only a minor effect on the cooling performance enhancement with a reduction of the thermal resistance from 0.11 K W [sup minus] 1 to 0.074 K W [sup minus] 1 at the highest heating power. In the second part of the study, heat transfer numerical simulations of the finned part of a cooler were performed and validated using experimental results. The output of the simulation is a 2-D temperature field, which is used as an input for the optimization methodology based on fin effectiveness and fin efficiency. Optimizing the fin geometry by removing unnecessary material yielded a 23% increase of the fin efficiency and decreased the weight of a fin by approx. 30%, proving the usefulness of the proposed methodology, which helps reduce costs, weight and development time of finned HP-based coolers

Waste-to-energy processes as a municipality-level waste management strategy

The escalating challenge of waste management demands innovative strategies to mitigate environmental impacts and harness valuable resources. This study investigates waste-to-energy (WtE) technologies for municipal waste management in Kočevje, Slovenia. An analysis of available waste streams reveals substantial energy potential from mixed municipal waste, biodegradable waste, and livestock manure. Various WtE technologies, including incineration, pyrolysis, gasification, and anaerobic digestion, are compared. The results show that processing mixed municipal waste using thermochemical processes could annually yield up to 0.98 GWh of electricity, and, separately, 3.22 GWh of useable waste heat for district heating or industrial applications. Furthermore, by treating 90% of the biodegradable waste, up to 1.31 GWh of electricity and 1.76 GWh of usable waste heat could be generated annually from biodegradable municipal waste and livestock manure using anaerobic digestion and biogas combustion in a combined heat and power facility. Gasification coupled with a gas-turbine-based combined heat and power cycle is suggested as optimal. Integration of WtE technologies could yield 2.29 GWh of electricity and 3.55 GWh of useable waste heat annually, representing an annual exergy yield of 2.98 GWh. Within the Kočevje municipality, this amount of energy could cover 23.6% of the annual household electricity needs and cover the annual space and water heating requirements of 10.0% of households with district heating. Additionally, CO2-eq. emissions could be reduced by up to 20%, while further offsetting emissions associated with electricity and district heat generation by 1907 tons annually. These findings highlight the potential of WtE technologies to enhance municipal self-sustainability and reduce landfill waste

Investigation of the scatter in reported pool boiling CHF measurements including analysis of heat flux and measurement uncertainty evaluation methodology

This study investigates the effect of experimental setup design factors on pool boiling CHF, compares spatial temperature gradient calculation methods and analyzes the uncertainty of heat flux and surface superheat. Reported CHF values on smooth copper surfaces, measured for saturated pool boiling of water at atmospheric pressure on flat horizontal samples, are highly scattered, which cannot be explained solely by the measurement uncertainty or the randomness of the boiling process. CHF data for 54 experiments from 47 publications is analyzed using regression analysis and ANOVA to determine which experimental setup design factors influence the CHF value. Methods for estimating the axial temperature gradient in a heating stem are compared using the Monte Carlo method and analytical nonlinear gradients. Heat flux values calculated using temperature measurements in a cylindrical copper heating stem together with either constant or temperature-dependent thermal conductivity and various temperature gradient calculation methods are compared. Overall heat flux and surface superheat measurement uncertainties are analyzed and the impact of contributing uncertainties including that of the thermal conductivity, temperature measurement and distance between thermocouples is reported

Evaluation of enhanced nucleate boiling performance through wall- temperature distributions on PDMS-silica coated and non-coated laser textured stainless steel surfaces

Nucleate boiling was examined on Joule heated stainless steel foils, functionalized by PDMS-silica coating and/or nanosecond-laser texturing. The nucleating bubbles and transient temperature fields were visual- ized through high-speed IR and video recordings. The differences in boiling performance were evaluated through wall-temperature distributions. Results confirmed that smooth surfaces require high activation temperatures and produce larger bubbles, while wall-temperature distributions display higher standard deviations, higher local superheats, and bimodal shapes. Similarly, relatively high activation temperatures were observed on the superhydrophilic surface, where the enhanced liquid replenishment on the active nucleation sites reduces the bubble departure diameters and prevents formation of local hotspots. Consequently, the analyzed temperature distributions have negative skewness and decreased standard deviation. The highest heat transfer coefficient was achieved on a laser textured surface with nonuniform wettability and multi-scale microcavities. Here, nucleation site density was as high as 200 cm-2 at 300 kW/m2 , while wall-temperature distributions demonstrated by far the lowest standard deviation. The temperature distributions also proved that annealed PDMS-silica coating significantly increased the thermal resistance of the entire heater. On the contrary, laser textured surfaces provided an even better boiling performance compared to coated surfaces and did not increase heater\u27s thermal resistance. This additionally endorses the coating-free, direct laser texturing method as a cutting-edge technology in the development of surfaces capable of significantly enhanced boiling heat transfer

Pattern geometry optimization on superbiphilic aluminum surfaces for enhanced pool boiling heat transfer

In this study, the optimal surface pattern of low and high wettability regions for enhanced boiling heat transfer is investigated using aluminum superbiphilic surfaces. Samples are fabricated by combining chemical vapor deposition of a fluorinated silane to turn them superhydrophobic and nanosecond laser texturing to render selected areas superhydrophilic. Triangular lattice pattern of superhydrophobic circular spots is utilized with spot diameters between 0.25 mm and 1.0 mm and pitch values of 0.5-2.5 mm. Pool boiling heat transfer performance of superbiphilic surfaces is evaluated using saturated water at atmospheric pressure. A strong wettability contrast is shown to be important in ensuring high heat transfer performance of wettability-patterned surfaces. Highest heat transfer performance is achieved using 0.5 mm diameter spots with a spot pitch of 1 mm and a corresponding superhydrophobic area fraction of approx. 23%. The optimal pitch value will provide a high density of potentially active nucleation sites but still allow for the development of the thermal boundary layer thus not inhibiting the activation of neighboring spots. The size of (super)hydrophobic spots appears not to have a major influence on the boiling performance when using the optimal spot pitch. The developed superbiphilic surfaces increase the CHF and provide greatly enhanced heat transfer coefficients especially at medium and high heat fluxes, making them suitable especially for high-heat-flux applications