Practical trainings, student assistants and theses

Drops, films, waves: validation experiments on horizontal two-phase flows (Id 201)

Bachelor theses / Master theses / Diploma theses / Student Assistant / Vompulsory internship

Foto: graphical abstract student position ©Copyright: Paul PorombkaThe optimization of phase change heat exchangers is closely linked to the prediction of the distribution of gas and liquid mass flows to the individual heat exchanger channels. Computational fluid dynamics (CFD) is becoming increasingly important in the design phase of these and other two-phase flow devices. In order to improve these numerical methods for the prediction of complex two-phase flows, the CFD model development is being advanced at the HZDR. For this purpose, reliable validation data is required, which is gather in HZDR-based experiments. To support this experimental work we are currently searching for student assistants. The candidate will be responsible for the following tasks:

• capture image data of horizontal two-phase flow using high-speed microscopy
• support of X-ray tomography measurement campaigns
• quantitative analyses of 2D and 3D datasets with respect to different validation parameters
• documentation of the results

Requirements of the respective university can be considered after consultation.

Department: Computational Fluid Dynamics

Contact: Porombka, Paul

Requirements

• experimental skills (ideally in the field of flow measurement)
• interest in fluid mechanics , especially multiphase flows (e.g. mechanical engineering, physics, process engineering)
• good command of a programming/scripting language (ideally Python or Matlab)

Conditions

• duration: 4-6 months
• start date: 1. June 2018
• compensation for expenses

Online application

Please apply online: english / german

Druckversion


A correlation of critical void fraction on/near the wall under the boiling crisis (CHF) (Id 134)

Student practical training / Master theses / Diploma theses / Student Assistant / Vompulsory internship

Nucleation boiling is commonly known as a most efficient way of transferring heat into a liquid, as it combines a large uptake of latent heat by the steam bubbles, convective transfer via bubble motion and a most effective mixing of the thermal boundary layers. However, when the heat flux becomes higher and reaches a critical value (CHF), parts of the heater surface become irreversibly covered by vapor and nucleation boiling turns into film boiling. In cases of power controlled heating this can potentially lead to a meltdown of the heater structure. Understanding and predicting the complex phenomena involved in the CHF is necessary for the efficient operation, safety and development of industrial applications like boiler, nuclear reactor, electronic/microchips system. However even with decades’ heavy investigations, the mechanism of forming CHF especially how the CHF is initiated from nucleation is still without a consensus explanation.
Recently a model of near critical heat flux (CHF-) is raised in our group, that is, the moment, when CHF is initiated, is inferred. This model gives to our opinion both a definite explanation on how CHF is initiated and secondly a quantitative value for the onset of CHF, which has been validated with a number of test cases from literature. Computational fluid dynamics (CFD) is an attractive way to support engineering design by 3D flow simulation in the future. It would be beneficial, if occurrence of CHF could be simulated with CFD codes. In last years an extended RPI model was developed and tested by ANSYS and HZDR CFD group together which requires the critical void fraction as a criterion. In the preliminary test, this value is set to 80% but which is confirmed should be case dependent.
The main tasks for this work are:
1. Simulate the multiphase flow in subcooled boiling process with standard RPI model where the CHF value calculated by CHF- model is considered as a input boundary condition.
2. Capturing the critical void fraction from the simulations.
3. Processing and analysis the captured results to generate an empirical correlations using MATLAB.
4. Applying the correlations to predict the boiling crisis with extended RPI model.

Department: Computational Fluid Dynamics

Contact: Dr. Ding, Wei

Requirements

1. Study of mechanical engineering, process engineering or similar
2. Knowledge of CFD
3. Basic knowledge of heat transfer
4. Knowledge of program/script language (e.g. MATLAB)

Conditions

Duration: >= 3 months
Begin: from now on

Online application

Please apply online: english / german

Druckversion