Systematic analysis of the extrusion process in 3D bioprinting is mandatory for process optimization concerning production speed, shape fidelity of the 3D construct and cell viability. In this study, we applied numerical and analytical modeling to describe the fluid flow inside the printing head based on a Herschel–Bulkley model. The presented analytical calculation method nicely reproduces the results of Computational Fluid Dynamics simulation concerning pressure drop over the printing head and maximal shear parameters at the outlet. An approach with dimensionless flow parameter enables the user to adapt rheological characteristics of a bioink, the printing pressure and needle diameter with regard to processing time, shear sensitivity of the integrated cells, shape fidelity and strand dimension. Bioinks consist of a blend of polymers and cells, which lead to a complex fluid behavior. In the present study, a bioink containing alginate, methylcellulose and agarose (AMA) was used as experimental model to compare the calculated with the experimental pressure gradient. With cultures of an immortalized human mesenchymal stem cell line and plant cells (basil) it was tested how cells influence the flow and how mechanical forces inside the printing needle affect cell viability. Influences on both sides increased with cell (aggregation) size as well as a less spherical shape. This study contributes to a systematic description of the extrusion-based bioprinting process and introduces a general strategy for process design, transferable to other bioinks.

This paper presents the first experimental results of the systematic investigation of forced convection heat transfer in scaled generic models of steam turbine casing side spaces with varied geometric dimensions under fully turbulent air flow. Data were obtained by two redundant low-heat measuring methods. The results from the steady-state inverse method are in good agreement with the data from the local overtemperature method, which was applied via a novel miniaturized heat transfer coefficient (HTC) sensor concept. All experiments were conducted at the new Side Space Test Rig “SiSTeR” at TU Dresden. The dependencies of the HTC distributions on the axial widths of the cavity and its inlet and on the eccentricity between them were investigated for Reynolds numbers from Re = 40,000 to 115,000 in the annular main flow passage. The measured HTC distributions showed a maximum at the stagnation point where the induced jet impinges on the wall surface, and decreasing values towards the cavity corners. Local values scaled roughly with the main flow Reynolds number. The HTC distributions thereby differed considerably depending on the dimensions and the form of the cavity, ranging from symmetric T-shape to asymmetric L-shape, with upstream or downstream shifted sidewalls.

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Purpose T-shaped cavities occur by design in many technical applications. An example of such a stator cavity is the side space between the guide vane carriers and the outer casing of a steam turbine. Thermal conditions inside it have a significant impact on the deformation of the turbine casing. In order to improve its prediction, the purpose of this paper is to provide a methodology to gain better knowledge of the local heat transfer at the cavity boundaries based on experimental results. Design/methodology/approach To determine the heat transfer coefficient distribution inside a model cavity with the help of a scaled generic test rig, an inverse heat conduction problem is posed and a method for solving such type of problems in the form of linear combinations of Trefftz functions is presented. Findings The results of the calculations are compared with another inverse method using first-order gradient optimization technique as well as with estimated values obtained with an analytic two-dimensional thermal network model, and they show an excellent agreement. The calculation procedure is proved to be numerically stable for different degrees of complexity of the sought boundary conditions. Originality/value This paper provides a universal and robust methodology for the fast direct determination of an arbitrary distribution of heat transfer coefficients based on material temperature measurements spread over the confining wall.

This paper presents the first experimental results of the systematic investigation of forced convection heat transfer in scaled generic models of steam turbine casing side spaces with varied geometric dimensions under fully turbulent air flow. Data were obtained by two redundant low-heat measuring methods. The results from the steady-state inverse method are in good agreement with the data from the local overtemperature method, which was applied via a novel miniaturized heat transfer coefficient (HTC) sensor concept. All experiments were conducted at the new side space test rig “SiSTeR” at TU Dresden. The dependencies of the HTC distributions on the axial widths of the cavity and its inlet and on the eccentricity between them were investigated for Reynolds numbers from Re=40,000 to 115,000 in the annular main flow passage. The measured HTC distributions showed a maximum at the stagnation point where the induced jet impinges on the wall surface, and decreasing values toward the cavity corners. Local values scaled roughly with the main flow Reynolds number. The HTC distributions thereby differed considerably depending on the dimensions and the form of the cavity, ranging from symmetric T-shape to asymmetric L-shape, with upstream or downstream shifted sidewalls.