Yes, there'll is valid, it means there will or there shall. Use it whenever the tone is casual: There'll be food at the party.
Formal writing? Prefer the expanded form: There will be a lot of traffic tonight. The full version sounds more measured on the page. In speech, texts and everyday notes, there'll is the natural choice.
When the setting is casual or conversational
Texting a friend about plans? Casual tone suits it: There'll be pizza when you get here. The contraction signals you're talking, not drafting a document.
Adding a quick note in a chat or message thread: Running late, there'll be a queue when I arrive. Contractions read naturally in everyday personal messages.
Leaving a voicemail: Hi, it's me. There'll be two of us for dinner, not one. Spoken messages take the contraction easily because you're saying the words, not editing them.
Writing dialogue or a personal letter: Don't worry, there'll be plenty of chairs. Fiction and informal writing take contractions comfortably, so nothing sounds off.
Composing a social post where you want a relaxed voice: There'll be cake at the launch tomorrow. It keeps the tone friendly without extra words.
When you're writing something formal
Submitting an academic paper, lab report, or formal application? Switch to there will be before you send it. Contractions are rare in scientific and scholarly writing, and will is usually written in full in formal register, so the expanded form matches what readers expect there.
Keep there'll for emails to colleagues, cover notes, or anything you'd actually say aloud. In those settings the contraction reads as natural speech, not as an error, so no rewrite is needed.