--- title: "Validation first: can CFD rank stirrer positions by kLa?" description: In a demanding 2 L kLa comparison at 1.5 vvm and 1400 rpm, we changed only the stirrer position and compared CFD against experimentally measured kLa. Even though the values were close, the simulation ranked the configurations in the right order and found a stirrer position that performed better than the standard setup. image: https://www.p4b.io/hubfs/website/insights/slides/kla-5.webp --- [![p4b](https://www.p4b.io/hubfs/raw_assets/public/p4b/images/p4b-logo-white.svg) ![p4b](https://www.p4b.io/hubfs/raw_assets/public/p4b/images/p4b-logo-navy.svg)](https://www.p4b.io/) Products [**pDS inserts**Large-scale conditions in your lab bioreactor](https://www.p4b.io/pds) [**Advanced CFD**Gradients, lifelines and cell experiences](https://www.p4b.io/cfd) [**DREAM-bio**Find the best geometry for your bioreactor](https://www.p4b.io/dream-bio) [**Ambr 250 vs. 2,000 L**Interactive CFD comparison of the mixing](https://www.p4b.io/compare) [Case studies](https://www.p4b.io/case-studies) [Insights](https://www.p4b.io/insights) [Events](https://www.p4b.io/events) [About](https://www.p4b.io/about) [Contact](https://www.p4b.io/contact) [Request a quote](https://www.p4b.io/contact#quote) [Home](https://www.p4b.io/) / [Insights](https://www.p4b.io/insights) CFD · 23 Oct 2026 # Validation first: can CFD rank stirrer positions by kLa? In a demanding 2 L kLa comparison at 1.5 vvm and 1400 rpm, we changed only the stirrer position and compared CFD against experimentally measured kLa. Even though the values were close, the simulation ranked the configurations in the right order and found a stirrer position that performed better than the standard setup. ![kLa experiment vs simulation: the simulation ranks the configurations in the right order, the previous user setup is marked](https://www.p4b.io/hs-fs/hubfs/website/insights/slides/kla-5.webp?width=900&height=900&name=kla-5.webp) Validation is useful when it survives a hard case. We took a 2 L lab-scale bioreactor and kept the reactor, rpm and gassing the same, changing only the stirrer position. At 1.5 vvm and 1400 rpm, this was far from a clean textbook case. The model had to handle turbulence, gas dispersion, bubble breakage and coalescence. Oxygen transfer was measured experimentally as kLa. The design question was simple: if operating conditions stay the same, can stirrer position alone improve oxygen transfer over the standard user configuration? The measured kLa values were close together, so the comparison was sensitive. A useful model had to detect small but relevant differences between configurations. The aim was not to match every absolute kLa value perfectly. It was to reproduce the trend that matters for the process: gas dispersion, bubble residence time, liquid circulation and the oxygen transfer trend. The result: even with close values, the simulation ranked the configurations in the right order and found a stirrer position that performed better than the standard setup. This is where CFD becomes practical for bioreactor design. It does not replace experiments, but it narrows the design space before every possible variant has to be tested: simulate the options, rank the configurations, test the best ones, improve the design. Validation first, optimization second. ![Same reactor, same rpm, same gassing, different stirrer position - 2 L kLa comparison at 1.5 vvm and 1400 rpm](https://www.p4b.io/hs-fs/hubfs/website/insights/slides/kla-1.webp?width=900&height=900&name=kla-1.webp)![The design question: three stirrer positions in the same 2 L vessel](https://www.p4b.io/hs-fs/hubfs/website/insights/slides/kla-2.webp?width=900&height=900&name=kla-2.webp)![Measured kLa for 14 stirrer configurations, values close together](https://www.p4b.io/hs-fs/hubfs/website/insights/slides/kla-3.webp?width=900&height=900&name=kla-3.webp)![What CFD needed to capture: gas dispersion, bubble residence time, liquid circulation, oxygen transfer trend](https://www.p4b.io/hs-fs/hubfs/website/insights/slides/kla-4.webp?width=900&height=900&name=kla-4.webp)![kLa experiment vs simulation: the simulation ranks the configurations in the right order, the previous user setup is marked](https://www.p4b.io/hs-fs/hubfs/website/insights/slides/kla-5.webp?width=900&height=900&name=kla-5.webp)![Why it matters: simulate options, rank configurations, test the best, improve the design](https://www.p4b.io/hs-fs/hubfs/website/insights/slides/kla-6.webp?width=900&height=900&name=kla-6.webp) Slides from the LinkedIn post - swipe through [← All insights](https://www.p4b.io/insights) Get in touch ## Let's find out how your process reacts to scale Tell us your system and target scale - we come back with a proposal for your insert set. 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