<p> The objective of this research is to investigate the effect of DC, AC and pulse wave applied voltage on two-phase flow patterns, heat transfer and pressure drop during tube side convective condensation of refrigerant HFC-134a in an annular channel. Experiments were performed in a horizontal, single-pass, counter-current heat exchanger with a rod electrode placed along the center of the tube. The electric field was applied across the annular gap formed by the electrode connected to the high-voltage source and the grounded surface of the inner tube of the heat exchanger. The electric field between the two electrodes was established by applying a high voltage to the central electrode. The high voltage was generated by amplifying the voltage output from a function generator. The flow was visualized at the exit of the heat exchanger using a high speed camera through a transparent quartz tube coated with an electrically conductive film of tin oxide.</p> <p> The effect of a 8 kV DC applied voltage was investigated for mass flux in the range 45 kg/m^2s to 160 kg/m^2s and average quality of Xavg= 45%. The application of the 8 KV DC voltage increased heat transfer and pressure drop by factor 3 and 4.5 respectively at the lowest mass flux of 45 kg/m^2s. Increasing the mass flux decreased the effect of electrohydrodynamic forces on the two-phase flow heat transfer and pressure drop.</p> <p> The effect of different AC and pulse wave applied voltage parameters (e.g. waveform, amplitude, DC bias, AC frequency, pulse repetition rate and duty cycle) on heat transfer and pressure drop was investigated. Experiments were performed with an applied sine and square waveform over a range of frequencies (2 Hz < f < 2 kHz), peak-to-peak voltages (2 kV < Vp-p < 12 kV) and DC bias voltage (-10 kV < VDc < 10 kV), and with an applied pulse voltage of amplitude 12 kV and duty cycle from 10% to 90%. These experiments were performed for a fixed mass flux of 100 kg/m^2s, inlet quality of 70%, and heat flux of 10 kW /m^2. For the same amplitude and DC bias, the pulse wave applied voltage provides a larger range of heat transfer and pressure drop control by varying the pulse repetition rate and duty cycle compared to the sine waveform.</p> <p> The effect of a step input voltage on two phase flow patterns, heat transfer and pressure drop was examined and analyzed for an initially stratified flow. The flow visualization images showed that the step input voltage caused the liquid to be extracted from the bottom liquid stratum toward the center electrode and then pushed to the bulk flow in the form of twisted liquid cones pointing outward from the central electrode. These transient flow patterns, which are characterized by high heat transfer compared to the DC case, diminish in steady state. The effect of the amplitude of the step input voltage and the initial distance between the electrode and liquid-vapour interface on the liquid extraction was investigated experimentally and numerically. At sufficiently high voltages, the induced EHD forces at the liquid-vapour interface overcame the gravitational forces and caused the liquid to be extracted towards the high voltage electrode. The extraction time decreased with an increase of the applied step voltage and/ or decrease of the initial distance between liquid interface and the high voltage electrode. The numerical simulation results were, in general, in agreement with the experimental results.</p> <p> The effect of pulse repetition rate of pulse applied voltage on two phase flow patterns, heat transfer and pressure drop can be divided into three regimes. At the low pulse repetition rate range, f < 10 Hz, the two-phase flow responded to the induced EHD forces, and liquid was extracted from the bottom stratum to the center electrode and then pushed back to the bulk flow in the form of twisted liquid cones. Increasing the pulse repetition rate in this range increased the repetition of the extraction cycle and therefore increased heat transfer and pressure drop. In the mid pulse repetition rate range, 10 Hz < f < 80 Hz, the extraction was not completed, which led to lower heat transfer compared to the lower pulse repetition rate range. In this range, the two phase patterns were characterized by liquid-vapour interface oscillations between the center electrode and the bottom stratum and liquid droplet oscillations which increased the momentum transfer and therefore pressure drop. Increasing the pulse repetition rate in this range decreased heat transfer and increased pressure drop. In the high pulse repetition rate range, f > 80 Hz, increasing the pulse repetition rate decreased both the interfacial and droplet oscillations and therefore decreased the heat transfer and pressure drop till the two phase flow patterns resembled that for an applied DC voltage. For the same pulse repetition rate, increasing the mass flux decreased the effect of EHD forces on heat transfer and pressure drop. The heat transfer enhancement ratio and pressure drop ratio increased with an increase of the duty cycle for the same pulse repetition rate of the applied voltage.</p> <p> Different combinations of pulse repetition rate and duty cycle of applied pulse wave voltage can be used to achieve different values of heat transfer and pressure drop. This can be very beneficial for heat transfer control in industrial applications. An advantage of such control is that it eliminates various measurements devices, control and bypass valves, variable speed pumps, fans and control schemes used in current technology for heat transfer and pressure drop control. The range of control of the ratio of the heat transfer coefficient to the pressure drop is from 8.24 to 20.56 for mass flux of 50 kg/m^2s and it decreased with increasing mass flux untill it reached 1.63 to 3.81 at mass flux 150 kg/m^2s.</p> / Thesis / Doctor of Philosophy (PhD)

Isothermal flow development, gap instability, and gap vortex street generation in eccentric annular channels have been studied experimentally. A representative paradigm of a flow in a highly eccentric annular channel was examined for a channel having an inner-to-outer diameter ratio d/D = 0.50 and an eccentricity e = 0.8 for a Reynolds number Re = 7300. Observation of the flow development has identified three distinct regions: the entrance region, the fluctuation-growth region and the rapid-mixing region. Weak quasi-periodic velocity fluctuations were first detected in the downstream part of the entrance region, and grew into very strong ones, reaching peak-to-peak amplitudes in the narrow gap that were nearly 60% of the bulk velocity. The dependence on inlet conditions, d/D, e and Re on the development and structure of flows was also investigated. Experimental conditions covered the ranges: 0 ≤ Re ≤ 19000, 0 ≤ e ≤ 0.9 and d/D = 0.25, 0.50 and 0.75. For Re < 7000, the Strouhal number, the normalized mid-gap axial flow velocity and the axial and cross-flow fluctuation intensities at mid-gap were found to increase with increasing Re and to depend strongly on inlet conditions. At higher Re, however, these parameters reached asymptotic values that were only mildly sensitive to inlet conditions. A map was constructed for the various stages of periodic motions vs. e and Re and it was found that, for e < 0.5 or Re < 1100, the flow was unconditionally stable as far as gap instability is concerned. For e ≤ 0.5, transition to turbulence occurred at Re ≈ 6000, whereas, for 0.6 ≤ e ≤ 0.9, the critical Reynolds number for the formation of periodic motions was found to increase with eccentricity from 1100 for e = 0.6 to 3800 for e = 0.9. The use of an empirically derived "mixing layer Strouhal number" permitted a universal description of gap vortex street periodicity in eccentric annular channels. This study has contributed to our understanding of the physical mechanisms that lead to gap instability and the development of a gap vortex street and the dependence of these flow phenomena on the channel geometry and the dynamic conditions of the flow.

Isothermal flow development, gap instability, and gap vortex street generation in eccentric annular channels have been studied experimentally. A representative paradigm of a flow in a highly eccentric annular channel was examined for a channel having an inner-to-outer diameter ratio d/D = 0.50 and an eccentricity e = 0.8 for a Reynolds number Re = 7300. Observation of the flow development has identified three distinct regions: the entrance region, the fluctuation-growth region and the rapid-mixing region. Weak quasi-periodic velocity fluctuations were first detected in the downstream part of the entrance region, and grew into very strong ones, reaching peak-to-peak amplitudes in the narrow gap that were nearly 60% of the bulk velocity. The dependence on inlet conditions, d/D, e and Re on the development and structure of flows was also investigated. Experimental conditions covered the ranges: 0 ≤ Re ≤ 19000, 0 ≤ e ≤ 0.9 and d/D = 0.25, 0.50 and 0.75. For Re < 7000, the Strouhal number, the normalized mid-gap axial flow velocity and the axial and cross-flow fluctuation intensities at mid-gap were found to increase with increasing Re and to depend strongly on inlet conditions. At higher Re, however, these parameters reached asymptotic values that were only mildly sensitive to inlet conditions. A map was constructed for the various stages of periodic motions vs. e and Re and it was found that, for e < 0.5 or Re < 1100, the flow was unconditionally stable as far as gap instability is concerned. For e ≤ 0.5, transition to turbulence occurred at Re ≈ 6000, whereas, for 0.6 ≤ e ≤ 0.9, the critical Reynolds number for the formation of periodic motions was found to increase with eccentricity from 1100 for e = 0.6 to 3800 for e = 0.9. The use of an empirically derived "mixing layer Strouhal number" permitted a universal description of gap vortex street periodicity in eccentric annular channels. This study has contributed to our understanding of the physical mechanisms that lead to gap instability and the development of a gap vortex street and the dependence of these flow phenomena on the channel geometry and the dynamic conditions of the flow.

No description available.

O presente trabalho apresenta um estudo associado à separação de gás para escoamento gás-líquido em um duto anular inclinado. Esse tipo de escoamento e geometria são encontrados em separadores de gás do tipo shroud invertido na indústria de petróleo quando utilizadas técnicas de bombeamento para a exploração de poços. A presença de gás livre a montante da bomba é uma das maiores limitações dos sistemas de bombeamento, por acarretar cavitação e falhas dinâmicas nos equipamentos. O presente estudo tem por objetivo garantir a separação total de gás livre a montante da bomba através da proposição de um separador do tipo shroud invertido em tubulação inclinada para aplicação na exploração de petróleo em poços direcionais. Um estudo experimental com ar e água como fluidos de trabalho a pressão quase atmosférica foi desenvolvido visando a compreensão da fenomenologia do separador shroud invertido. Foram observados escoamentos em duto anular do tipo: bifásico gás-líquido ascendente, monofásico em canal livre e bifásico gás-líquido descendente devido ao fenômeno de aeração; foram coletados também dados de eficiência de separação em função do ângulo de inclinação, vazão do líquido e queda de pressão entre o shroud e a saída do tubo de produção. Encontrou-se uma variável de extrema importância no fenômeno de separação até agora não reportada na literatura: o nível do anular interno do shroud (NAI). Um modelo fenomenológico que prevê a separação total do gás foi desenvolvido a partir da interpretação dos fenômenos físicos observados experimentalmente. Uma correlação inédita para a modelagem do fenômeno de dissipação de energia cinética turbulenta vinculado à separação do gás é proposta. O modelo foi validado qualitativamente com dados da literatura e ajustado com os dados coletados neste trabalho, mostrando boa concordância. / This paper presents a study associated with gas separation in an inclined gas-liquid annular-duct flow. This type of flow and geometry are found in shroud-inverted gas separators applied to petroleum industries when using pumping technique for oil production. High void fraction at the pump suction of is one of the most important limitations of the SCP technique, causing cavitation and dynamics failures in the equipment. The present study aims to provide a solution for the total gas separation through the use of an innovative inclined inverted-shroud separator for directional wells. An experimental study, where air and water at near atmospheric pressure constituted the working fluids, was carried out to understand the phenomenology of the inclined inverted-shroud separator. Different annular-duct flows were observed: upward gas-liquid flow, single-phase open channel flow, downward gas-liquid flow due to the phenomenon of aeration; also new data of separation efficiency were collected as a function of inclination angle, liquid flow rate and pressure drop between the shroud and production pipe outlet. One of the most significant findings is that the liquid level of the inner annular channel (NAI) of the shroud is a very important variable in the phenomenon of separation; so far this was not reported in the literature. Based on the observations, a phenomenological model that predicts total gas separation is proposed. A new correlation for the modeling of the dissipation of turbulent kinetic energy associated with the gas separation is presented. The model was qualitatively compared with available data from the literature and quantitatively adjusted against the new experimental data obtained in this work, and the agreement was quite good.

O presente trabalho apresenta um estudo associado à separação de gás para escoamento gás-líquido em um duto anular inclinado. Esse tipo de escoamento e geometria são encontrados em separadores de gás do tipo shroud invertido na indústria de petróleo quando utilizadas técnicas de bombeamento para a exploração de poços. A presença de gás livre a montante da bomba é uma das maiores limitações dos sistemas de bombeamento, por acarretar cavitação e falhas dinâmicas nos equipamentos. O presente estudo tem por objetivo garantir a separação total de gás livre a montante da bomba através da proposição de um separador do tipo shroud invertido em tubulação inclinada para aplicação na exploração de petróleo em poços direcionais. Um estudo experimental com ar e água como fluidos de trabalho a pressão quase atmosférica foi desenvolvido visando a compreensão da fenomenologia do separador shroud invertido. Foram observados escoamentos em duto anular do tipo: bifásico gás-líquido ascendente, monofásico em canal livre e bifásico gás-líquido descendente devido ao fenômeno de aeração; foram coletados também dados de eficiência de separação em função do ângulo de inclinação, vazão do líquido e queda de pressão entre o shroud e a saída do tubo de produção. Encontrou-se uma variável de extrema importância no fenômeno de separação até agora não reportada na literatura: o nível do anular interno do shroud (NAI). Um modelo fenomenológico que prevê a separação total do gás foi desenvolvido a partir da interpretação dos fenômenos físicos observados experimentalmente. Uma correlação inédita para a modelagem do fenômeno de dissipação de energia cinética turbulenta vinculado à separação do gás é proposta. O modelo foi validado qualitativamente com dados da literatura e ajustado com os dados coletados neste trabalho, mostrando boa concordância. / This paper presents a study associated with gas separation in an inclined gas-liquid annular-duct flow. This type of flow and geometry are found in shroud-inverted gas separators applied to petroleum industries when using pumping technique for oil production. High void fraction at the pump suction of is one of the most important limitations of the SCP technique, causing cavitation and dynamics failures in the equipment. The present study aims to provide a solution for the total gas separation through the use of an innovative inclined inverted-shroud separator for directional wells. An experimental study, where air and water at near atmospheric pressure constituted the working fluids, was carried out to understand the phenomenology of the inclined inverted-shroud separator. Different annular-duct flows were observed: upward gas-liquid flow, single-phase open channel flow, downward gas-liquid flow due to the phenomenon of aeration; also new data of separation efficiency were collected as a function of inclination angle, liquid flow rate and pressure drop between the shroud and production pipe outlet. One of the most significant findings is that the liquid level of the inner annular channel (NAI) of the shroud is a very important variable in the phenomenon of separation; so far this was not reported in the literature. Based on the observations, a phenomenological model that predicts total gas separation is proposed. A new correlation for the modeling of the dissipation of turbulent kinetic energy associated with the gas separation is presented. The model was qualitatively compared with available data from the literature and quantitatively adjusted against the new experimental data obtained in this work, and the agreement was quite good.