R134a Flow Boiling Heat transfer on an Electrically Heated Carbon/Carbon Surface

With the increase of heat flux densities following the Moore’s law, electronic cooling challenge is focused on the high heat flux to be dissipated by the operating fluid and more and more efficient heat spreaders, dissipators, and compact heat exchangers are in great demand for various applications. Considering the device efficiency, the boiling heat transfer ensures very high heat transfer coefficients, which can even be improved via specific surface treatments that have been shown to be very effective. In particular, several authors, experimentally demonstrated the interesting enhancement capabilities of microparticles coatings on the Critical Heat Flux. Furthermore, the recent work on nanoscale domain has led to new concepts for surface modification. In the last decade, nano-structured materials (i.e. nanowires coatings, nanoporous layers, Carbon Nano Tube arrays, etc.) have been proved to enhance the boiling heat transfer. Unfortunately, almost all of this kind of surface treatments fail when scaled up to industrial implementation because of the relatively high costs and complex operations involved. Furthermore, compactness and lightness of cooling systems are becoming even more challenging design constraints leading the research efforts towards new light and efficient materials. In this scenario, the Carbon/Carbon material appears to be a viable option for future thermal management devices because it exploits interesting properties having a low density and a high thermal conductivity; moreover, it is already used in many industrial applications where it is shaped in various forms even complex. This paper presents the experimental measurements carried out during flow boiling heat transfer of R134a on a Carbon/Carbon surface. The test section with the Carbon/Carbon sample, is electrically heated from the bottom and it is instrumented with 18 wall thermocouples to monitor the temperature distribution at an imposed heat fluxes of 50 kW m-2, and refrigerant mass flow rates from 50 to 200 kg m-2 s-1, at constant saturation temperature of 30 °C. The sample is tested in a new experimental facility built at the Nano Heat Transfer Lab of the Department of Management and Engineering of the University of Padova especially designed to study the flow boiling heat transfer process on innovative materials and enhanced micro- and nano-structured surfaces.

R134a Flow Boiling inside a 4.3 mm ID Microfin Tube

The energy and environmental performance of refrigeration and air conditioning machines are commonly described by their Total Equivalent Warming Impact, so called TEWI, which is defined as the sum of the indirect and direct emissions. The direct emissions are related to charge inventory of the system and to the type of refrigerant used, while the indirect emissions basically depend on the system energy performance. Even if there is a strong interest in the new low-GWP refrigerants, the traditional HFC fluids, with huge GWPs, are still widely used in the refrigeration and air conditioning equipment. For this reason, there is a still strong demand of innovative solutions which can be implemented with the current fluids and then applied to the new ones, when there will be the final phase-out of the HFCs. From this standpoint, looking at the TEWI index, the charge minimization and the system performance optimization represent the main targets of the innovation to cope with the environmental challenges. Since the early 1970s, traditional microfin tubes have been widely used in air and water heat exchangers for heat pump and refrigerating applications because they have been demonstrated to significantly improve the heat transfer performance during both in-tube condensation and boiling. The possible downsizing of microfin tubes could lead to more efficient and compact heat exchangers and thus to a reduction of the refrigerant charge of the systems and to an overall improving of their performance. Nowadays, large manufacturers are exploring the possible use of mini microfin tubes and there is a strong interest in understanding the heat transfer and pressure drop behaviours of this enhanced tube. This paper presents the R134a flow boiling heat transfer and pressure drop measurements inside a mini microfin tube with internal diameter at the fin tip of 4.3 mm. This study is carried out in a new experimental facility built at the Dept. of Management and Engineering of the University of Padova. The microfin tube was brazed inside a copper plate and electrically heated from the bottom. Sixteen T-type thermocouples are located in the copper plate to monitor the temperature distribution during the heat transfer process. In particular, the experimental measurements were carried out at constant mean saturation temperature of 30 °C, by varying the refrigerant mass velocity between 200 kg m-2 s-1 and 800 kg m-2 s-1, the vapour quality from 0.1 to 0.95, at four different heat fluxes: 15, 30, 60, and 90 kW m-2. The experimental results are presented in terms of two-phase heat transfer coefficient, onset dryout vapour quality, and frictional pressure drop as a function of the operative test conditions

nanoparticle deposition during cu water nanofluid pool boiling

The present research activity aims to rigorously investigate nanofluid pool boiling in order to definitively assess this as a technique for controlled nanoparticle coating of surfaces, which can enhance the nucleate boiling performance. This paper presents preliminary nanoparticle deposition results obtained during Cu-water (0.13 wt%) nanofluid pool boiling on a smooth copper surface. The tests were run in an experimental setup designed expressly to study water and nanofluid pool boiling. The square test sample block (27.2 mm × 27.2 mm) is equipped with a rake of four calibrated T-type thermocouples each located in a 13.6-mm deep holes drilled every 5 mm from 1 mm below the top surface. The imposed heat flux and wall superheat can be estimated from measurement of the temperature gradient along the four thermocouples. The samples are characterized by scanning electron microscopy (SEM) to analyse the morphological characteristics of the obtained thin, Cu nanoparticle coating

Adiponectin levels in the serum and cerebrospinal fluid of amyotrophic lateral sclerosis patients : possible influence on neuroinflammation?

BACKGROUND: Adiponectin (APN) is a key player in energy homeostasis strictly associated with cerebrovascular and neurodegenerative diseases. Since APN also belongs to anti-inflammatory-acting adipokines and may influence both neuroinflammation and neurodegenerative processes, we decided to study the APN levels in amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases. METHODS: We assessed APN levels by ELISA immunoassay in both the serum and cerebrospinal fluid of a cohort of familial and sporadic ALS patients, characterized by normal body mass index and absence of dysautonomic symptoms. The screening of serum APN levels was also performed in patients affected by other neurological disorders, including fronto-temporal dementia (FTD) patients. Means were compared using the non-parametric Wilcoxon test, and Pearson's or Spearman's rho was used to assess correlations between variables. RESULTS: In the whole ALS group, serum APN levels were not different when compared to the age- and sex-matched control group (CTR), but a gender-specific analysis enlightened a significant opposite APN trend between ALS males, characterized by lower values (ALS 9.8\u2009\ub1\u20095.2 vs. CTR 15\u2009\ub1\u20099.7 \u3bcg/ml), and ALS females, showing higher amounts (ALS 26.5\u2009\ub1\u200911.6 vs. CTR 14.6\u2009\ub1\u20095.2 \u3bcg/ml). This sex-linked difference was significantly enhanced in familial ALS cases (p\u2009 64\u20090.01). The APN levels in ALS cerebrospinal fluids were unrelated to serum values and not linked to sex and/or familiarity of the disease. Finally, the screening of serum APN levels in patients affected by other neurological disorders revealed the highest serum values in FTD patients. CONCLUSIONS: Opposite serum APN levels are gender-related in ALS and altered in several neurological disorders, with the highest values in FTD, which shares with ALS several overlapping and neuropathological features. Further investigations are needed to clarify the possible involvement of APN in neuroinflammation and neurodegeneration. Possible involvement of APN in neuroinflammatory neurodegenerative diseases

The contact angle of nanofluids as thermophysical property

Droplet volume and temperature affect contact angle significantly. Phase change heat transfer processes of nanofluids – suspensions containing nanometre-sized particles – can only be modelled properly by understanding these effects. The approach proposed here considers the limiting contact angle of a droplet asymptotically approaching zero-volume as a thermophysical property to characterise nanofluids positioned on a certain substrate under a certain atmosphere. Graphene oxide, alumina, and gold nanoparticles are suspended in deionised water. Within the framework of a round robin test carried out by nine independent European institutes the contact angle of these suspensions on a stainless steel solid substrate is measured with high accuracy. No dependence of nanofluids contact angle of sessile droplets on the measurement device is found. However, the measurements reveal clear differences of the contact angle of nanofluids compared to the pure base fluid. Physically founded correlations of the contact angle in dependency of droplet temperature and volume are obtained from the data. Extrapolating these functions to zero droplet volume delivers the searched limiting contact angle depending only on the temperature. It is for the first time, that this specific parameter, is understood as a characteristic material property of nanofluid droplets placed on a certain substrate under a certain atmosphere. Together with the surface tension it provides the foundation of proper modelling phase change heat transfer processes of nanofluids