Competition between cooling and shortwave radiation in shaping near-surface convection in pelagic waters of Lake Geneva

When lake surface waters above the temperature of maximum density 4◦C cools, the verysurface fluid parcels become denser than their neighbours below. The latter process leads to a gravitationally unstable density distribution which may trigger ‘free convection’, i.e. the upper denser waters look for their equilibrium position somewhere down in the water column. This phenomenon usually occurs at night time, when the atmosphere is colder than surface waters. However, during daytime, it may happen that the lake surface cools—releasing heat to the atmosphere—while shortwave radiation penetrates through upper waters. In this scenario, the near surface waters of the photic zone can host an unstable layer due to the net surface cooling, whereas waters beneath it experience radiative heating and gravitational stabilization. Here, we investigate the competition between shortwave radiation and surface cooling when wind is low or negligible. Thus, depending on the relative intensity of surface cooling and shortwave radiation, the upper water column undergoes different gravitationally unstable density distributions and potentially convective regions. The objective of this Master Project is to investigate, describe, and characterise such convective regimes. To do so, the heat equation controlling temperature evolution of the upper water column subject to surface cooling and penetrative radiative heating is investigated. In parallel with in-situ observations, mathematical and numerical modelling allow to understand the phenomenon taking place. The results allow to infer how the relative importance of surface cooling and penetrating shortwave radiation may affect vertical heat and mass transfer within the lake and between the lake and the atmosphere.

Les circulations d'eau dans les massifs cristallins alpins et leurs relations avec les ouvrages souterrains

The present study has been achieved within the framework of the scientific program AQUALP, supported by the EVIAN Foundation. Its theme is the study of the hydrogeology of the Alpine crystalline. The AQUALP programme consists of two co-coordinated parts : while the "source" part, lead at the Savoy University, studies natural circulation towards springs, the "work" part, which concerns this study, has focused on the interaction between civil engineering works and underground waters. The approach adopted is that of a field study during which water inflows encountered in various Alpine underground works and replaced in their geological contexts were followed up as from the discharge, the physico-chemical and the isotopic chemistry point of view. A tracer test and numerical simulations of underground flows were also undertaken. Main results The summary of the hydrogeological data in the numerous existing underground works allowed to calculate, on the basis of simple analytical solutions, the distribution of the hydraulically conductivity of the massifs crossed. These results show that the deeper one goes the more the hydraulical conductivity decreases. They bring us to specify the thickness of the decompressed zone, i.e. about 500 m. This zone strongly influences the underground flow in these massifs. Presence of mica inhibits the hydraulical conductivity in conferring a more plastic comportment to the rocks during past tectonic phases. The temperature of the water inflows encountered is a complex function of the thickness of the overlying rock and the quantity of waters circulating in the massif. Therefore it constitutes a precious indicator, during the excavating of underground works, of the presence of possible more permeable sections in formations to come. A numerical simulation of such a case has been undertaken and shows the possibility of using this tool to improve the expectation of water inflow in the work. The temperature study as well as the application of the chalcedony geothermometer to the entire of the inflow studied has shown the existence of ascending flow systems, fed by precipitation. Waters circulating in the Alpine massifs have a chemical composition strongly influenced by the reservoir rock. Chemical analyses of major and trace elements have allowed to distinguish waters issued from the various petrographies encountered. One must first of all underline the complementarity of major and trace elements analyses. The discriminating power of these two types of analyses does not apply to the same type of rocks. The major structures hydraulically conductive in crystalline Alpine massifs have been defined. One can mainly quote isolated fractures, strongly fractured and tectonised zones, kakirite zones, the decompressed zone, the down-bending zone, contacts between distinctive hydrogeological units and large metasedimentary structures. Hydraulical and physico-chemical characteristics of waters associated to each type of defined structures have been determined. These structures are straightened and induce water circulation close to the vertical, from the surface towards the work. The infiltration zones of waters determined on the basis of the orientation of major structures generally coincides with the altitudes calculated on the basis of isotopic analyses of waters (0-18). Some anomalies have nevertheless been put into evidence. They are mainly relative to circulation in the decompressed zone or in overlying Quatemary beds. Relationship between surface waters (barrier lakes or rivers) and underground works have also been put in evidence on the basis of the interpretation of isotopic results. Numerous water inflows are poor in tritium and attest of the time of transit through to the work in the order of a number of decades, in relation with the weak permeability observed in depth. These waters are often rich in sodium, because of the alteration of silicates contained in rocks. More recent waters have been encountered in shallow depths, in the decompressed zone or in association with a great geological fault. The examples of the interaction between underground works and springs are systematically linked to important water inflows in the works either through a decompressed zone of the massif, or through a particularly transmissive zone connected with the surface. These examples show that in this fissured environment, extremely divided into compartments and anisotropic, the drying up of springs is not linked with the progressive lowering of a water table along side the work but at punctual and localized inflows. A conceptual model of the hydrogeology of the Alpine crystalline is proposed in this study. It is accompanied with an interaction model between a work and the natural middle. It must help the project manager to foresee the water inflows in underground works and to adopt the optimal solution to minimize the impact of the work on water resources.

EPFL1998

Numerical Modeling of Annular Laminar Film Condensation in Circular and Non-Circular Micro-Channels under Normal and Micro-Gravity

Stefano Nebuloni

A theoretical and numerical model to predict film condensation heat transfer in mini, micro and ultra micro-channels of different internal shapes is presented in this thesis. The model is based on a finite volume formulation of the Navier-Stokes and energy equations and it includes the contributions of the unsteady terms, surface tension, axial shear stresses, gravitational forces and wall thermal conduction. Notably, interphase mass transfer and near-to-wall effects (disjoining pressure) are also included. This model has been validated versus various benchmark cases and versus published experimental results from three different laboratories, predicting micro-channel heat transfer data with an average error of 20 % or better. The conjugate heat transfer problem arising from the coupling between the thin film fluid dynamics, the heat transfer in the condensing fluid and the heat conduction in the channel wall has been studied and analyzed. The work has focused on the effects of three external wall boundary conditions: a uniform wall temperature, a non uniform wall heat flux and single-phase convective cooling. The thermal axial and peripheral conduction occurring in the wall of the channel can affect the behavior of the condensate film, not only because it redistributes the heat, but also because the annular laminar film condensation process is dependent on the local saturation to wall temperature difference. When moving from mini to micro and ultra-micro channels, the results shows that the axial conduction effects can become very important in the prediction of the wall temperature profile and they can not be ignored. Under these conditions, the overall performances of the heat exchanger become dependent not only on the fluid properties and the operative conditions but also on the geometry and wall material. Results obtained for steady state conditions are presented for circular, elliptical and flattened shape cross sections for R-134a and ammonia, for hydraulic diameters between 10 µm and 3 mm. Microscale condensation finds applications in heat pipes and compact heat exchangers for electronic equipment or spacecraft thermal control, in automotive condensers, in residential air conditioning and in refrigeration applications: the influence of the steady or unsteady gravitational field and of the inertia forces on the flow field and consequently on the heat transfer performances is investigated allowing the model to be applied as a design and optimization tool for enhanced heat exchangers.

EPFL2010

On the Adaptation of Building Controls to the Envelope and the Occupants

David Daum

The sun is the biggest known source of energy in our solar system. We feel its strength when it gets hot during the the day and we notice its absence during the night when we feel cold. So as to be less dependent on the sun as an energy source, we implemented additional heating and cooling sources to maintain the temperature within a comfortable range. The downside to this is that the majority of energy consumed within the housing sector is used up on the heating and cooling alone. To profit from the vast energy source of the sun we propose a user-adaptive and building-adaptive blind control for residential buildings, that is implemented in prefabricated modules for facade renovation. User-adaptive means that it is the occupant who is responsible for the temperature control within the home. Building-adaptive, in this context, means that the temperature control is established automatically without any user input. Through the evaluation of occupant queries we have shown that a general measure for thermal comfort is not possible for all occupants. Consequently, there is a need for a personalized measure of thermal comfort. In order to create this the occupant enters votes via the interface; from this we deduced statistically the probability of comfort relative to the indoor temperature. According to the profile the control sets its target temperature. The profile steadily adapts the user's preferences and through this we can also capture seasonal changes in comfort temperature. This guarantees that at each point in time the control system knows the desired temperature and is taking action to achieve it. The adaption to the building is achieved with the fitting of a simple thermal building model with data collected by the sensors of the control system. We showed that the monitored data sufficiently fits the model. With the help of the simple model we evaluated different control strategies and optimized them according to the thermal profile. For our performance tests we conducted computer simulations as well as a 6-month field study. For the simulations, a specific test bed was suggested that would assess the saving potential, which can then be compared to the performance of the tested control. Results showed that the suggested control system is capitalizing on most of the achievable energy savings and thermal comfort. A 6-month field study in the LESO-PB building was carried out to test the impact on energy demand as well as comfort under real conditions. It appeared that the automatically controlled office needed only approximately 50% of the average heating energy that was used in the manually controlled offices. Furthermore, the probability of thermal comfort was, on average, 10% higher in the automatically controlled offices when compared to those that were controlled manually.

EPFL2011